I usually just "know" when it's time, especially when I've been training a lot. I think in the past year I've been through about 4 pairs of road shoes and just 1 pair of trail shoes (since I can only get out to the trails on weekends.) I tend to err on the side of caution though for road shoes, maybe 300-400 miles if I had to guess, because I have a heavy footfall and need some cushioning on the roads.