how often do i have to change the belts on my 1998 328 i convertible

Never

They have a timing chain that never needs changing

usually cars have 2 types of timing belts: chain and rubber belts. All japanese, german (including BMW)........ have chain timing belts and they are very hard to break to. To answer your question, you would never need to change your timing belt (believe me, they would never break) unless you want to waste some money the dealer (believe me, i have a car garage and i am telling you the truth.)

Every 50,000 miles according to BMW's own recommended service interval manual.

I used to hear the same things (regarding never changing the timing belt) that the other poster mentioned but mine did break at 203,000 miles and the entire engine seized - needed new engine.