You don't need to dispel it - if you fall under the effect of a spell while already being under the effect of the spell, the more powerful effect overwrites the weaker one. On average it takes 24 seconds of in-game time to get yourself a full strength False Life active.That's assuming that the DM actually runs 6-8 discrete combat encounters, all at least an hour apart. (Or else the warlock doesn't mind spending a spell slot to dispel the effect between encounters.)
Anyway, to get back to the business at hand . . .