fabian2,
Each round have 20 min, since there is a count down that ends the match, unless you change the time limit for something bigger.
I play with original time limit (20 minutes) am i get tired after one or two matchs, so i rarely plays more than that.
I also made maps for this game, and was normal for my self, in the past, to work more than 12 hours straight on maps.
May be is a good idea to try to play the match in the 20 minutes time limit, changing the number of enemies as a way to better choose dificult.
Since you cant set individual bots skill level, another way to change dificult is changing the number of enemie bots.