Depends on which parts of the act you consider to be sex. If you include foreplay, cuddling, petting, kissing, etc, then it should last between 30 mins to 1 hour.
If you're talking just about the intercourse, then that should last anywhere between 1 to 5 minutes. 5 minutes is actually quite a long time for it to last, approaching excessive. Think about how long a 5 minute video on Youtube is, you almost have to think about whether to watch it or not because it's so long. It's the same thing for sex.