I think if you have a good program that is well thought out, you will vary your round times to achieve various objectives. For example, for positional sparring, you would expect the round times to be shorter. If you're working on endurance, round times would need to be longer. To improve cardio, maybe you're not just taking the round time into consideration, but the rest times as well. At open mat, we just put 5min on the clock, with one min rest in between rounds, but this is the only time we have standard round times.

So, during regular practices do you set round times arbitrarily?

No. Nothing's arbitrary. Everything is well planned out, like I said. As a coach, I plan the entire training session prior to arrival. We very rarely, if ever, say, 'OK bro, let's just roll'. That's just open mat, in my opinion. Instead, sparring rounds are very carefully planned in a way that emphasizes what I, as a coach, want to accomplish that day. For example, I might start everyone in top mount, as that's the position we practiced in class. It's a hard position to keep, so having 5min rounds might not make sense. On the other hand, if I'm noticing that we, as a team, have trouble performing well for 5 minutes, then it makes sense to have longer rounds. It's definitely not arbitrary (randomly/at whim). Does this make more sense?