Should I just assume that a Warlock with the Fiendish Vigor invocation starts every combat with 8 temporary hp?

Question

Warlocks have access to this invocation:

Fiendish Vigor

You can cast false life on yourself at will as a 1st-level spell, without expending a spell slot or material components.

False Life is a 1 action spell with a 1 hour duration that does this:

Bolstering yourself with a necromantic facsimile of life, you gain 1d4 + 4 temporary hit points for the duration.

Given the negligible cost in casting it (outside combat), wouldn't every warlock just keep casting it round after round until they got 8 temporary hp which would then last 1 hour? And then do it again when those expired. And again. And ...

Answer 1

Sure, it can't hurt.

Assuming that the player agrees (and I see no reason for them not to), yes, you can just assume that a warlock with that invocation starts with their buffs on.

I played a 3.5 game where all of us were using highly optimized, self-buffing characters who would stack buffs on ourselves before every combat. It was a waste of time for everyone to announce their buffs at every encounter, so the DM just assumed that we had them on at the start of each non-surprise encounter, and we subtracted the resources ourselves. We never had any issues with this system.

Just don't forget about it--having a spell constantly on like that means that they will show up on detect magic, for example, and there might be situations where constant obvious spellcasting might draw unwanted attention.

Answer 2

This is a topic that came up in our recent campaign.
We agreed that just like a person doesn't walk around repeatedly holding their breath in case they fall in water, a fighter doesn't walk around with his weapons drawn, my Warlock doesn't walk around expending the (even relatively little) effort to keep recasting a spell on herself 'just in case'. If nothing else, calling upon my pact over and over without any real reason is a draw on my patronage.
If on the other hand, we see combat coming, then I cast the spell as we ready (just as everyone is able to draw their weapons) and I roll 1d4+4 (once).
edit: For example, if we're walking into a magical tower where the Lich was last seen, I'm casting False Life. If we're strolling through the woods on 5hour march to our destination, I'm not recasting it regularly.

Why not always start with 8 hitpoints? Simple. I only know the spell worked. Not how well it worked. Whatever my first successful cast gives me, I have no reason to believe I can do it better by repeating the process.

Thanks to @IcyFire's comment. We play that PCs don't know the numbers of their stats. We know a clean, hard hit vs a glancing blow. We know when healing has made a substantial difference vs when it only seemed to stem the bleeding. So I acknowledge that this part is probably not relevant to all.

Answer 3

The Invocation "fiendish vigour" states You can cast False Life on yourself at will as a 1st-level spell, without expending a spell slot or material components.

So it wouldn't even take a turn (action/bonus action) you can literally cast it as many times as you want in one turn with no care for the action economy, you could practicaly say you just take 8 less damage from all sources.