For me, it depends on what I'm doing. Give me something tedious, and I can barely focus to save my life. If it's something I'm well suited to, I can do it for hours and hours, resenting even the short breaks my body forces me to take in order to get something edible from the refrigerator. Maybe you just haven't really thought about which activities you have the most stamina for?

I'd find it hard to do math for a whole 90 minutes, but I can write, do visual art or do emotional support for hours at a time. Not sure how long I can flow while programming - the boss said I have to take breaks. I think I've gone at least two hours.