There's a balance to be sure, and there's some learning about yourself that is necessary. For instance, there's a critical difference between the feelings "I'm burning out and do not want to do this task" and "This task is boring".
It's possible you're feeling lazy; it's also possible that you're closer to burnout than you realize and that is your brain/body telling you that you need a break. Only you can decide which is which. There are consequences to being wrong either way, but, well, c'est la vie, non?
I've had some success maintaining some longish work hours, even under the American-style "vacations are for people who live in other countries and don't have babies" regime, for a very long time with this basic approach because only rarely will I force it, and when I have to for some reason I always budget some days that I will simply take off from computing entirely.
