This is like asking "when should I have the cylinders reground or the oil pump replaced. A timing chain is not like a timing belt in that it is not subject to 80,000 mile replacement. So, keep your oil changed every 3000 miles and drive on. Good luck !!!
I have a 94 grand Cherokee and they have a timing CHAIN not a belt so I'd say you're gettin screwed! the water pumps are not very sturdy in these cars so that I could understand. I've never done my chain and have 298000km Good luck>