I'm on 134K now on the original exhaust. Mine is in pretty bad shape - the centre box has been filled on two occasions with chemical metal and the straight section has two 'mr sheen' cans, split and smeared with exhaust paste and then jubilee clipped to the existing pipework.
I have no leaks and the MOT station simply gave me a small advisory for 'repairs made'.
I have seen a cat back full system for £60 on eBay, so will change when uneconomical to repair.
Also I guess it depends upon your journeys - I drive a 40 milee round trip, so the exhaust does get a chance to dry out on a daily basis.
Regards,
Steve