I think USAT recommends a minimum of 5 minutes. Sometimes I've seen as little as 3 or 4, and sometimes up to 7 or 10. Length affects it somewhat, but the biggest reason is just to get the bikes spaced out reasonably. Sometimes things are done like: send the elites out, then wait 7 minutes to give them some room, then go to 5 minutes for all of other waves. Or send the first timers wave off, give them 15 minutes, and then go to 5s.
Most of the races I've done so far have been between 3 and 5 mins.
I did, however, do a sprint with a 2-minute gap between waves. That was waaaayyy too short, especially since the wave behind me was 29-39 males. It was like the strongest wave there, and they ate my wave up (I was in the youngest women's wave). I would have appreciated a little more time to get away from them .
The ones I have done have been between 5-7minutes. The first one I did ended up being closer to 10 because one of the swimmers in front of us was struggling. It was nice to see the race director let the waves go when the water in front was clear instead of just starting everybody 5 minutes apart.