I am looking at the false life spell, and am a bit confused.  It is cast at will and has an hour duration.  What would prevent a player from casting it all the time, and at that point, doesn't it basically function as a permanent (if slightly variable) increase to hit points?

Here is the spell text...

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

What are your thoughts?

What normally stops that? It is a level 1 spell that consumes spell slots, so it would take at least 12-16 spell slots to keep that up during a typical adventuring day.

What stops that for a warlock with the Fiendish Vigor invocation? Nothing, it is intended that they can do this all day to always have 1d4+4 temporary hitpoints going into an encounter. 

What would prevent a warlock from casting it every three seconds, and obtaining hundreds of extra hit points?

Temporary hit points don't stack. 

Thanks

You also can only benefit from a spell once at a time - if two people cast Guidance on you at the same time, you're still only rolling 1d4 once and it's all gone.

So, even if temporary hit points did stack, re-casting the same spell also prevents stacking anyway.

Quote from Epipactis >>

What would prevent a warlock from casting it every three seconds, and obtaining hundreds of extra hit points?

 I did sort of short cut where if my warlock had 1 minute to keep casting false life, he would do so until rolling max value.

DxJxC, my DM has used Cyb3rM1nd's point to justify not letting me always just reat 1d4+4 as 8. It isn't a concentration spell, so I'm not sure whether you can voluntarily dismiss it (is there a rule on that somewhere?), you might be stuck with your crappy 5 thp roll until an hour expires or something punches you. 

But if it were up to me, yeah, I just treat the warlock as always having 8 THP going into a fight, it isn't a particularly powerful invocation choice, no reason to begrudge the player a few hp unnecessarily. 

It's reasonable to assume you can eventually max it out :

"If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. "

Where is that from? Yeah, that sounds good if it’s real.

Quote from Chicken_Champ >>

Where is that from? Yeah, that sounds good if it’s real.

 https://www.dndbeyond.com/sources/basic-rules/combat#TemporaryHitPoints

4th paragraph.

This is true, warlocks with that specific invocation should always have 8 temp hp outside of combat, as long as they have a minute or so to cast it until it "works."