you are meant to change them every 40k (ish), it will do serious damage to your engine if you dont and the engine snaps (alot more than the cost of a cam belt)
A Cinq Haynes manual gives you a good run down on how to do it. I would suggest changing the tensioner as well, as you will be working with it anyways.
For anybody that isn't sure, a "safe engine" is one where if the cam belt snaps, the valves will not hit the pistons, which will happen in most engines, and therefore ruining it.
............................I wouldn't fancy testing it on either engine style though....
I got the cam belt changed on my Sei today. It has done 37200 miles