when they start to look like a circular saw blade, it's time to change them. when they wear far enough for the track to slip and you're out of adjustment on the tensioner, you shoulda changed them 6 months ago..
around here sprockets will last through 3 or 4 sets of tracks... by then the rear rollers start to look like rounded off sprockets as well....
when i worked at a deere dealer down in florida, we serviced a LOT of small dozers. new chains and sprockets had a life of about 500 hours, by then the drive side of the track bushings were worn, and the adjuster was at the end of it's travel
we'd pull the track chains, turn the pins and bushings, flip the sprockets from side to side, and them run them until destruction. it wasnt unusual to have 4 or 5 of them sitting outside the shop waiting on the track pins and bushings to get turned and reassembled...