I've played combat on servers with 10 min, 15 min, and 20 min.
And I have to say, 15 min is by far the best, for several reasons.
1. Its more fun to play for 15 minutes. By the 10-13 min mark all of your mediocre to excellent players on the server will be level ten. Meaning the high tech usuage gets a lot of use, but it's not boring to the point where you spawn in, run at the hive, shoot it, and die.
2. It helps marines. Even though, sadly, I have no clan play experience to back this up (no leagues do combat...), I must say that on pubs the longer the game is the better the chance of marines winning. I don't know why this is, but it is.
3. Did I mention it's a lot more fun?
15 minutes still feels like a struggle, gives plenty of time to level up, and keeps it more balanced between aliens and marines.
I say the default time limit should be 15 minutes. Anyone else agree with me?