Right 325is = m20 = belt. My advice is if you care about your car, change it every year, that way you don't even have to think about it. It's very cheap todo, and you can do your valves while your in there.
BMW rates them @ like 60k, what celsdogg said, but I'd change them before 50k. Blown timing belt pretty much means your looking at dropping 500 dollars in, atleast.