I have an sl2 w/ 135,000 miles on it and would like to know when the timing belt or chain should be replaced?

Alot depends on how regularly you change the oil. A car that is taken care of will not need one for its entire life.

Normally you will not need a new chain unless something happens that provokes a replacement. The chain is built quite well :)

Thanks for the insight! I have only had this for about 4000 miles, but the only thing I really did to it was replace the tranny valve body and tune it up, it runs like a top