How frequent can you use an invocation?

I have Fiendish Vigor Invocation.  How often can I I cast it?  

If in Combat and  I am level 1, I assume I can cast it as a magic action, therefore I can't swing a weapon or cast anything else?

Can I cast it every turn?  If I can, can I ever get above 12 temporary points?

How long do the points last if I am not in combat?  I cast it when I wake up before breakfast,   No combat all day so when I go to sleep, I still have those points?   Does a long rest remove them?   

So, the ability says:

You can cast False Life on yourself without expending a spell slot. When you cast the spell with this feature, you don’t roll the die for the Temporary Hit Points; you automatically get the highest number on the die.

It says you cast the spell, which means it is indeed a magic action, and thus consumes your action. If you can cast a spell as a bonus action, you can still do that. (And, since it doesn't use a spell slot, your BA spells are unrestricted.)

Since it doesn't use a spell slot, and doesn't place any restrictions on how often it can be used, nothing stops you from using it every turn.

Everything else is answered in the Temporary Hit Point rules.

You can't swing a weapon or cast another spell normally if you use a magic action to cast false life through fiendish vigor. But it does not cost a slot so if you have a bonus action spell to cast or bonus action attack like from a monks martial art feature you can use those, haste would also get you another attack. And I am sure options I am not thinking of but optimizers know by heart are available. I can't think of any at level 1, but again optimizers might.'

edit to add the obvious hex option. but if there is another bonus action spell on your level 1 list it can be used as well. And say summoning a weapon with pact of the blade is also an option. 

Quote from MyDudeicus >>

You can't swing a weapon or cast another spell normally if you use a magic action to cast false life through fiendish vigor. But it does not cost a slot so if you have a bonus action spell to cast or bonus action attack like from a monks martial art feature you can use those, haste would also get you another attack. And I am sure options I am not thinking of but optimizers know by heart are available. I can't think of any at level 1, but again optimizers might.'

edit to add the obvious hex option. but if there is another bonus action spell on your level 1 list it can be used as well. And say summoning a weapon with pact of the blade is also an option. 

 First level Warlock spells with a casting time of 1 Bonus Action include Hex, Expeditious Retreat, and Armor of Agathys.