Almost none. I haven't listened to the radio at all in around 6 or 7 years. I find that there isn't anything on the radio which I like at all.
I do know what's going on in the mainstream because my girlfriend has a very different musical taste than I do, and she listens to Pop, Rock, Top 40, etc...
I do enjoy some quality Hip Hop, but none of the stuff I like would ever be played on the radio. Also, my girlfriend likes a small amount of Hip Hop, but she can't stand the Hip Hop that I listen to. She would call it "boring." I would call top 40 "engineered."
I mostly listen to Drum & bass, Breaks, IDM, House, Electro, and other Electronica. There are very few radio stations that play Electronica in the USA. It simply is not mainstream. I did once hear a station playing some House in New York City. That was cool, although I do not live there.
Before I got into Electronica I listened to Alternative Rock, Punk, Grunge, and Metal. I did listen to the radio then.
I also listen to some Classical, Jazz, and Reggae. I guess those count as a bit of mainstream, but not compared to the more popular genres.