At 15K, it's a good idea to check and adjust the valves. They should not be too far from spec.

To accurately check the timing chain stretch requires a dial indicator to be setup on the #1 intake valve, after the valve lash is removed. The engine is rotated until the valve lifts .080". Then the reading is taken from the degree wheel on the crankshaft damper and compared to a known spec. IIRC it's 11 degrees on an existing (worn) chain. Almost all chains that have been in service for awhile won't meet this spec. The question is how much the chain exceeds this value. The manual calls for using an offset key to adjust the valves back to spec, however, there is some disagreement as to whether replacement of the chain is a wise idea if the stretch exceeds a certain amount.

Some folks run the chains over 250K without replacing them. Others replace the chain on principle after a certain number of miles. There is no consensus.