Well, this is a tricky question for me. If we're talking about washing my hair, I rarely do it on loose hair so my wash time (even when it includes DCing) usually doesn't go over an hour. There's no detangling involved coz I don't allow my hair to get tangled in the first place.
When putting in extensions, I can take up to ten hours, but I tend to do it as I feel like it so that could stretch over a couple of days if I don't feel like doing it in one sitting.
Then I have HIH disease so I'm always playing in my hair. If I am not doing any work with my hands and I'm sitting watching TV, you can bet your bottom dollar, I'm redoing a braid or a twist. That could be ongoing so not sure if I can even come up with a number. "Indefinitely" is probably the best response for that.
I don't think I ever let my hair exhaust me. Once I braid or twist it, I forgetaboutit and will only work on a few sections at a time. And even during the install, I do not do it longer than I feel like it. If it's taking too long for my liking, I put whatever's left in big plaits and don a hat or headwrap for as long as it will take for that excitement of doing hair to return to me. Hair doesn't rule; I do!
ETA: I'm natural 4B