The charmed condition from Create Thrall lasts until otherwise removed, the ability is used on someone else, or a Remove Curse is cast upon the target. These abilities have no limit on daily use other than Hypnotic Gaze being once per creature per long rest.
Importantly, Create Thrall’s charmed condition does not break under normal conditions, like taking damage. And if you’ve reached a level 14 warlock, you likely have a 20 in Charisma, which, if you have advantage on attempts to persuade the target to not attack you at that point and proficiency, you’re likely going to succeed.
Finally, putting a few levels into wizard gives a warlock extra spell slots for longer encounters, which can save them. They don’t want to be using 5th level spell slots on something like, Silvery Barbs, for example, despite how good it is. I’d argue taking 3 levels in wizard is even better, since your potential spell slot access doubles.
This is very much a build that can take down major threats single-handedly, and is designed to function as such. A little bit of a meme build, but an enjoyable thought experiment.
It’s situational. I know that. But that’s why it’s fun to come up with these really weird build ideas. This one just so happens to be more effective against the players than the enemies they fight, for the most part, part of why I noted it would make a great villain.