Prolonging spells with a Magic Circle

I was looking at Circle Spells from Heroes of Faerun. I had a doubt about the Prolong option:

Prolong

When you cast a spell that has a duration of 1 minute or longer, you can increase the duration of the spell depending on the number of secondary casters contributing to the spell, as detailed in the table below.

Each secondary caster contributing to the spell must expend a spell slot (no action required). If the spell fails, these spell slots aren’t expended.

Secondary Casters	The Spell Increases Duration By …
1–3	1 hour
4–6	8 hours
7+	24 hours

Let's say a cleric upcasts Aid at lev 3 and their companions, a warlock a druid and a pally all spend a level 1 slot, does it means each secondary caster prolongs the spell by 1 hour to 11 hours or do they only increase the spell duration to 9 h like if only 1 of them had joined? 

I tend to think it is the latter, but then why even bothering adding 2 or 3 secondary spellcasters if 1 is enough?

Also does circle magic allow for 1h (and 1m) Bless and Mirror Image with a single secondary spellcaster spending a lev 1 spellslot?

It's 1 hour total for 1-3 secondary casters, not 1 hour for each caster. So in your example the spell would last for 9 hours. (In this particular case it probably isn't worth it.)

This option can be applied to any spell with a duration of 1 minute or longer. Bless and Mirror Image both have a duration of 1 minute, so they are both eligible. Either spell could be stretched out by up to 24 hours with enough people, though with Bless someone would need to be concentrating on it the whole time.

Quote from wagnarokkr >>

though with Bless someone would need to be concentrating on it the whole time.

 Could a group have a trusted priest cast bless, raise its duration to 8 or 24h .. and then leave him in a safe place and enter the dungeon?

Quote from Jayce_Danathar >>

Quote from wagnarokkr >>

though with Bless someone would need to be concentrating on it the whole time.

 Could a group have a trusted priest cast bless, raise its duration to 8 or 24h .. and then leave him in a safe place and enter the dungeon?

Sure, I don't see why not. The spell has no requirement that the targets have to remain near the caster.

Quote from Jayce_Danathar >>

Quote from wagnarokkr >>

though with Bless someone would need to be concentrating on it the whole time.

 Could a group have a trusted priest cast bless, raise its duration to 8 or 24h .. and then leave him in a safe place and enter the dungeon?

 Better hope that priest doesn’t stub their toe in that safe location. 😀

If I were DMing, I’d definitely let you do it, but I’d also have there be some chance something happens (like the stubbed toe, or just falling asleep) that might break the concentration. It’s a clever idea, and I’d want you to get some use from it, but there would be no guarantee it would last the whole duration. Off the top of my head, roll a d20 1/hour and on a 1 it goes away. Something like that. 

Quote from Jayce_Danathar >>

Quote from wagnarokkr >>

though with Bless someone would need to be concentrating on it the whole time.

 Could a group have a trusted priest cast bless, raise its duration to 8 or 24h .. and then leave him in a safe place and enter the dungeon?

 This would actually be a really cool way to reward the party for allies they make throughout the campaign as they head towards the final confrontation of the campaign. Have the different groups they have allied with pool their resources to buff the party with long-lasting circle-cast buff spells.

Quote from wagnarokkr >>

It's 1 hour total for 1-3 secondary casters, not 1 hour for each caster. So in your example the spell would last for 9 hours. (In this particular case it probably isn't worth it.)

This option can be applied to any spell with a duration of 1 minute or longer. Bless and Mirror Image both have a duration of 1 minute, so they are both eligible. Either spell could be stretched out by up to 24 hours with enough people, though with Bless someone would need to be concentrating on it the whole time.

 If that's correct, why would you ever use 2 or 3 secondary casters? you'd be burning spell slots for no reason.

Quote from AidenCaterwall >>

Quote from wagnarokkr >>

It's 1 hour total for 1-3 secondary casters, not 1 hour for each caster. So in your example the spell would last for 9 hours. (In this particular case it probably isn't worth it.)

This option can be applied to any spell with a duration of 1 minute or longer. Bless and Mirror Image both have a duration of 1 minute, so they are both eligible. Either spell could be stretched out by up to 24 hours with enough people, though with Bless someone would need to be concentrating on it the whole time.

 If that's correct, why would you ever use 2 or 3 secondary casters? you'd be burning spell slots for no reason.

The 1-3 range is likely to allow balance and verisimilitude to align. Having the tiers be 1 secondary cast for 1 hour, 2 for 8 hours, and 3 for 24 hours would be overpowered. However, having it be exactly 1 caster for 1 hour, exactly 4 for 8 hours, and exactly 7 for 24 hours would seem narratively odd. So they went with ranges of 1-3, 4-6 and 7+ to reconcile those two elements. Yes, it's a given you're probably going to employ the fewest possible casters, but it makes sense there should be the option to employ more. This way the casting option aligns with the other circle options.

Quote from Davyd >>

Quote from AidenCaterwall >>

Quote from wagnarokkr >>

It's 1 hour total for 1-3 secondary casters, not 1 hour for each caster. So in your example the spell would last for 9 hours. (In this particular case it probably isn't worth it.)

This option can be applied to any spell with a duration of 1 minute or longer. Bless and Mirror Image both have a duration of 1 minute, so they are both eligible. Either spell could be stretched out by up to 24 hours with enough people, though with Bless someone would need to be concentrating on it the whole time.

 If that's correct, why would you ever use 2 or 3 secondary casters? you'd be burning spell slots for no reason.

The 1-3 range is likely to allow balance and verisimilitude to align. Having the tiers be 1 secondary cast for 1 hour, 2 for 8 hours, and 3 for 24 hours would be overpowered. However, having it be exactly 1 caster for 1 hour, exactly 4 for 8 hours, and exactly 7 for 24 hours would seem narratively odd. So they went with ranges of 1-3, 4-6 and 7+ to reconcile those two elements. Yes, it's a given you're probably going to employ the fewest possible casters, but it makes sense there should be the option to employ more. This way the casting option aligns with the other circle options.

 yeah but having 2 or 3 casters is not only pointless, but burns spell slots unnecessarily. It's no more odd that having casters 2 and 3 literally not contribute at all. Every other option scales per caster. It makes no sense that this one does not. If it was an increase per caster, then at 4 it would get a big increase, then 7 would be an even bigger increase. You wouldn't need to get 24 people together.

There's also nothing that says it's limited to 24 hours. If you circle cast something that lasts 24 hours with 1 other person, it'll last 25 hours. There are no limits on any of the other spells besides the range increase, Augment. Expand will let you increase it by as much as you have the casters for. It just does it 10 feet at a time, so it's not a worry that you'll cover the whole planet. Augment also doesn't cost the secondary caster's spell slots. It makes no sense that you would have to burn spell slots for no benefit. And the chart says 7+. If it was set to 24, why would you put a plus on there? that's just wasting even more spell slots.