I think (and this could be self-limiting) that powering through a lack of motivation is a limited process, but if you can line up the motivation (in the fulfilling sense, not a pep-talk) then there's no real limit.

I think this lines up with Marissa Meyer's thoughts on why burnout isn't "real", i.e. that burnout, in the pervasive "abandon all hope ye who enter" form, is not over-exertion but under-motivation. This matches my own experience where I feel energised by working hard on things I care about, and worn out by working even trivially on things I don't.