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.