Okay, the twilight cleric had me thinking, if they have unlimited range dark vision, then the limit on how far they can see if based on how far they can physically see. In a dungeon, that's not really a problem, we can usually see to the end of a cooridor. But how far can characters in D&D physically see, roughly, if nothing obstructs their line of sight?

There aren't really rules for this. [Apparently in DMG instrad of PHB]

Depending on the character's height the horizon will be about 3 miles away (about 2 for small races). Obviously, anything that far away will have a small angular size, so checks may be required to spot something specific. And there will most likely be something obscuring their view that is closer, like trees or buildings.

There is probably no rules for this because it doesn't really matter in a game mechanics sense.

Chapter 8 of the DMG; Visibility Outdoors:

When traveling outdoors, characters can see about 2 miles in any direction on a clear day, or until the point where trees, hills, or other obstructions block their view. Rain normally cuts maximum visibility down to 1 mile, and fog can cut it down to between 100 and 300 feet.

On a clear day, the characters can see 40 miles if they are atop a mountain or a tall hill, or are otherwise able to look down on the area around them from a height.

Thanks.

I know it was a useless question, but I feel like it could be useful to keep in my back pocket.

It's not a useless question, it's one that comes up often with overland travel and the like.

Pro-tip: If you ever have a question that you think the rules doesn't cover, check Chapter 8 of the DMG. There's a lot in there that's invaluable for DM'ing.

Quote from Davedamon >>

It's not a useless question, it's one that comes up often with overland travel and the like.

Pro-tip: If you ever have a question that you think the rules doesn't cover, check Chapter 8 of the DMG. There's a lot in there that's invaluable for DM'ing.

 Good tip. I'll remember that before putting my foot in my mouth. (I did use the search, but we all know you can't find anything useful that way).

Here's a fun update: I'm using what I just learned in this to help with the same question. 

I'm creating an encounter where the party has to go inside an enemy safe house. Imagine a SWAT style raid. I'm allowing the party to position "sniper/spotters" on select 2 two story buildings so I was wondering how much a character could see from across the street, how to factor in for angles/field of view, etc and this was actually helpful. Cheers. 

It's not useless when you have a sorlock using Eldritch Lance, Spell Sniper and Extended Spell to make their Eldritch Blasts have a range of 1200ft 

Welcome to wanting eyes of the Eagle. The DMG distances are based on real values based on the curve of the earth - a 6’ human ( eyes at @ 5’10”) standing on the sea shore will see out to a horizon about 2.5 miles (13,200 ft). The “average adult male is about 5’9” (eyes at @ 5’7”) and so sees a horizon a little closer - hence the 2 mile listing as a good average for everything from gnomes to Goliaths and Dragonborn. 

Dang, look at you bring real world stuff into a fantasy game!

Says the gal who does the same thing all the time.

But of course - no matter how wild the fantasy folks basic thinking is real world - then they overlay the fantasy. Understanding the real world lets you know just which sections are fantasy. Enjoy the fantasy but know where it begins and ends.