Every time one sees an (or an image of an) attractive female (or male, if they're homosixual). I guess it depends on the mood / situation / that particular days' turnout. I'd put it at around 10-15 times on average. But it's not the active thought of making love that clouds our every action. It's more of a subconscious thing. It could happen while eating, shopping, working out, or anything.
"Should I get these new jeans? Yeah, that chick would bang me if I did."
"Should I eat at Subway or Charlie's? The chick at the counter of Subway is cuter."
"Which treadmill should I get on? That one next to the one the hot chick is on."
So.. yeah, it's more of an undertone to our daily routine than anything else.

Okay, I see. In that case, maybe once every few days for me. I'm hardly a representative sample, I probably have a dampened libido (chronic pain, depression, medications - take your pick).
At most, possibly 5 times a day, but only if I'm really horny. :blush2:
my "raging hormones" phase lasted about 6 months, and only happened after I got put on the pill for my PCOS about 5 years ago.
EDIT: I just don't think about it that much because I don't really get any benefit from thinking about it, so... I don't think about it. :shrug: