I read an article here on FR that said current US radio programs play as many as 25 minutes of ads in a 1 hour show. LOTS of interruptions. Europe, so I have heard, places its commericals at half hour intervals, not breaking up the programs as much. How many minutes of ads in 1 hour, though, I don't know.
When I was in France in October 1999, I remember FM radio sounding EXACTLY like American FM (except for the language, of course).
I'll always associate "Mambo Number Five" with that trip, because I heard it so d**m many times.