Depends on the rider and whether you are talking steel or aluminum.
I would guess that your stock steel sprockets should last somewhere around 12,000 miles if properly taken care of.
Around this time you should consider changing both front and rear as well as your chain.
Mileage may vary...
My chain lasted exactly 10k, but was skipping big time the week before I changed it out. I currently have almost 13k on the stock rear steel sprocket, very little wear so far. I would imagine the rear would easily last to 20k. I just changed to a 14t front sprocket for gearing, even though it wasn't worn. I ride pretty hard, wheelie a couple times a day, over 1k a month, rarely lube the chain (bad habit).