After a few (number depends on the situation) it takes longer and longer to get there. It is highly dependent on my state of arousal, how long it has been since the last time I had a “session” and what it is that is making me cum.

If I’m masturbating to porn, the porn has to get better and better or eventually I won’t get there.

If I’m with a partner it depends heavily on what we’re doing and their ability to continue.