The N20 does not have a timing belt. It has a timing chain. There is no set service interval. Most N20 timing chain replacements are done due to pre-existing problems such as a noisy startup due to timing chain elongation or crank/cam timing correlation faults due to timing chain elongation. We have a two-part video series that talks about both aforementioned problems and the procedures related to the repair (click the video button in the product photos) and there is a service bulletin from BMW talking about those issues as well, SI B B11 03 17, which is publicly available on the NHTSA's website.