I'm just saying, 20 to 30 minutes (a series of usually 3 minute rounds) seems to be what most pros do according to this book. It also depends on how hard you are going at it, I guess. If you are just doing some light contact shadow boxing type stuff on the bag I'm a sure a guy could go all day. If you are laying into it like you mean it then you aren't going to last that long. I'm not sure why guys who have sparring and focus mit training and whatnot to do would waste a lot of time training mildly on the heavy bag. I use it like that sometimes when I don't have a partner but that's in addition to short intense rounds.
* The best link I could find on Mike Tyson's training routine: