No set formula here. Last time I dropped it was years ago--since then, the two beaters that I got I did not put collision on from the get-go, so that was easy. Right now, my two "new" cars have years to go, probably 4 years and 100k before I'll think about it on them. Thing is, I'll probably trade out of them into new/newer cars before they age much past that (can't predict the future, but that fits the pattern for us).