Back when I actually HAD partners, it was about 5 or 6 times a week, provided the partner was in the area. For example, when I was up at university, I really didn't get to have sex (even if I'd wanted to, I was too busy with classes, work, and trying to have a social life). But when I'd come home for long weekends and such, I'd have a decent amount of sex with my FWB.
Also, when I was in a polyfidelity-type relationship just prior to college, I had sex a little more often, usually once a day, sometimes more. But I chose 2-6, age 18-25, since that's my most common situation.