It all depends on how they are wearing, climate, etc. I have the originals on my 98 M roadster, that's 8 years and 50k. They look fine, no cracks, rips, or tears. If the belt starts squeeking (dry), or fraying replace it. If I were getting ready for a long trip I may replace them, but I have had belts last up to 100k. Hot dry climate may last less.