None, I've never gotten sick of a song, and I listen to songs over, and over and over and over again. I can't think of any that I've been sickened by, if I love them enough to play them on repeat then chances are, they'll never die for me, even if I have a little break from a band for a while then return to them. My top 10 tracks on last.fm all have 400-1k+ plays each and I still sit and listen to them regularly.