IMO, it should be for 5-10 min. Any longer than that is superfluous really.
It's like human boxing: boxers don't spar for 12-15 rounds (36-45 min) every time they train. They usually only spar for 3-6 rounds (9-18 min). A dog gets plenty of schooling in that short amount of time, without putting undue fatigue or risking too great an injury.
If you have any kind of an eye at all for a dog, you can just feel if they're into it or not. A dog that has an intense, happy look on his face ... and is busy trying to do his job ... is just getting some work in to hone his skills, which is all he should be doing as a young, developing athlete.
No reason whatsoever to make it a long, drawn-out affair ... the indepth reasons for which I have stated pretty clearly in my book.
The only reason a human fighter "goes the distance" is when the money is on the line ... and so too should it be for the dogs
Jack