I've seen speedpaints that take up to 25+ min but I feel the most patience/chunk of time people have is max 20 min. Usually the ones I like the most are between 12-15 min. The ones that are less than 5 always go much too fast for me to follow.
Additionally, if you're thinking of recording some digital art speedpaints I might recommend you have both the screen of the entire picture as well as the part you're zoomed in/working on. Some people have only the zoomed in part and with the speed-up time the rapid in-and-out gets a bit dizzying in my opinion. I like having the "god view" box to watch as well. If that sounds interesting to you I'll find one set up like that to send to you for reference. Again, that's just my opinion and totally can be disregarded. I'm not sure what programs let you do that or not since all I do is traditionally. Which in those cases we avoid that issue all together.
I definitely agree on the time. None of the speedpaints that I've done (so far) have been more than 15 minutes but I was thinking about recording my comic process, and I was worried that might take more time. I'll probably have to do more editing if that's the case, but it'll be worth it.
I'm not too worried about the zooming aspect, I don't think I zoom in and out that much with my digital art... Hopefully...
I feel watching any speedpaint you do would be totally worth it If I gotta break it down into chunks of sitting down time I will watch it all! So I wouldn't worry too much Sounds like you got a good feel for your goal