I'm not sure to be honest. It is not about the time but about the speed as well. You can do 10 laps in 15 minutes but you can also do 20 laps in 15 minutes. I never worry about how long I swim for. But I set a number of laps lets say 15, then I time myself. If I do it in 10 minutes then next time I want to do it in 9 minutes and so on. Keep trying to increase your speed. Then after a while you increase the amount. You do 20 lengths and you time yourself and repeat the cycle.
Because you can get a better work out in 15 minutes than 30 minutes depending on how you swim and the number of laps you swim.
Also remember to never just stop at the end. Do 1 or 2 laps very slowly your calm yourself down.
When I do it, I sometimes get to a point where I feel like I am burning while underwater. I also feel like I am sweating while I am underwater. You will be tired as hell but it will be worth it.