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.