I have over 7,000 miles in higher mile bike with a Praxis big ring (46 cross bike) and it shows no signs of wear yet. Probably one of the cheapest per mile replacement item on the bike I'd imagine. I have changed the chain on that bike just once and the cassette is still going strong as well. The "general" rule of thumb I've heard is every 3 chains replace the cassette, every 2-3 cassettes replace the rings, assuming you clean & lube the chain regularly. Unless shifting is poor, obviously, then replace immediately.