I would say on average about one a month. It really depends, sometimes I can go two or three months between other times I may get two or three in a month. Also, guys do not have quite as many guy specific products to chose from, add to that that guys only make up 25% of the reviewing members and you can see that many factors come into play. lol
I get some gems and I get some coal, if you know what I mean. Funny though, some of them pieces of coal turn into real diamonds after some proper testing. So you get some good and bad, not a bad deal when it's all relatively free. Plus your review of said questionable product may save another person from purchasing a bomb.
You can limit some of the questionable products by making selections in your "My product material preferences" in the "Settings" section of your account page. At least you will not get a material that you may not like.