Depends on how hungry I get and how much energy you can supply - you're not exactly immortal, and therefore have a limited supply of energy you can give without dying - don't worry, I never draw nearly enough for that, and I'll be pacing it so you don't ever even get close to that point (been there several times myself, it's awful).

Of course, the fact that you have the willing mindset speeds up the process - I didn't know what was happening, only that there was a presence draining my lifeforce, and my panic and fear kept me from transforming for quite a while.