Sex varies depending on the people having it. If ur with an experienced guy, you guy can have sex for a full 30 mins easy.
If the guy is new, he may not last 5 mins, or even 1 minute for that matter.
It simply boils down to this, the more experience the better you can get at sex and the longer you can learn to make it last.
That by the way is by no means a reason to be a slut. If u want ur first time to be okish, get a guy who knows what hes doing and go with that.
Remember, sex is only as good as the ppl having it, and lasts only as long as you can make it last!!