About 4 races, I usually replace the bushing 1 time. There about $5 and very easy to replace and then the next time replace the entire drum about $65.(Recently I found aftermarket drums from Andrews Racing on eBay for $32) I find that if I go more then 8 races without changing both sprockets and the chain I end up breaking something sometimes the chain gets a bad spot earlier then that.
One other thing I keep a lot of is the shim/spacer between the crank and clutch drum I bought about a 1/2 dozen of the ones for a 10 tooth and another 1/2 dozen for the 11 tooth drums and then filed them all to different thicknesses and put each one through a tie rap with a piece of tape with the size written on them so when I change a bushing or drum I have them on hand for a easy quick swap.
