Casual sex with a person can be either one-time only or regular. However, by statistics, most of times when modern folks use the word hookup, they mean from 15 minutes to 1 night of the intercourse.
The getaway with a hottie throughout the weekend is longer than a few hours, but it’s a hookup, still. But we do not count too long while hookuping at the nightclub briefly in a cozy corner.
So, it all depends on the situation, place, and casual partners’ roles. No one would typically call a hookup something that lasts longer than a one-night-stand, although we do see our hookups for more sex.
Let’s be clear, even dating is short and temporary on a today scene. So, by the logic, casual affairs are much shorter than that. It matters if you want to look contemporary enough to your buddies.
If all is well and we like our hookup, it’s totally normal to keep on seeing each other about twice a month without ever promising to continue. It can last up to three months for still being called casual.