This is stuff most people would call "work" because it does take effort, it makes me a better developer and much of it ties somewhat directly into problems I'm facing at work. But for me it's winding down after work, and recharging for the next day.
Sometimes I have so much to do at work that I need to just read fiction or play around in the darkroom to relax. But this is not the norm -- it's happened twice in the last decade, and lasted for about a month.
So for the question of how many days I can do it? On average 1795.
Considering just the general area of IT, maybe 80 %? I get to do product development which is my interest, but there are also many things which I would like to be different, like purpose of product, organisational size, leadership philosophies, etc.
Now -- why do you ask?
I can work all waking hours on passion projects (including the boring grind - not just the infatuation and ideation phase) as long as it's self directed. Finding work opportunities that aren't orthogonal to long term areas of interest and don't feel like accumulating useless experience and constructing a parallel career is hard.
Ok but you're literally describing a burnout cycle. Some people would prefer to just skip that part and have a completely sustainable working pace.
Doesn't matter how much I love an activity I can hardly do it all time for more than few days.