I was reading about dist belt replacement on PCA.org, a while back. Of note is that on '95 (I think) and 964s there is not a venting mechanism and ozone created by the spark ionizing the air causes faster disintigration of the belt, and that if the belt goes VERY bad things can happen (IIRC one of the plugs can continuously spark causing serious damage in the combustion chamber) and it is difficult to detect until the damage occurs.

The support and knowledge on this board rocks! Best $32 I ever spent.

 

...and to add more infos, my car (96) has a vent, and 120.000km (+- 75K miles)
The belt was still looking good. Only the profile of the "teeth" was rounded. To my knowledge, it has never been changed before.

 

I've heard that the 964 did not have the vent tube by my 95 993 C2 does have it.