As long as you want it to!

There is no set rule about the beginning and the end. You can even take a break in the middle. Sex doesn't need to be defined by a finish line - when one or both people orgasm - you can keep going after or stop before that happens.

If you are just talking about penis in vagina sex, one study showed that the average for cis-het couples is about 5 and a half minutes. But it ranged from 33 seconds to 44 minutes!