S&S says to check every 10K miles. If you have the spacers installed on the front cover that will greatly improved the life of the belt as heat is its main enemy. Mine is going on 30K miles and aside from the small buildup of fibers, which I wipe out, it looks good. It should be good for 50K miles when a rebuild of the motor would be a good idea anyway.
The rear belt should be good for 100K miles unless a rock finds its way in there and holes it.
These things are tough!
