I'm not sure what model you are enquiring about and your profile doesn't indicate what model(s) you own, but the answer is nearly the same anyway. I would do a search on the subject and review what has been said in the past.
Basically, the chains don't stretch. They elongate with wear (double chains included). There are methods for checking the amount of elongation based upon where the timing marks are when the camshaft is in a given position. If you hear the very distinctive noise of a broken piece of chain, it needs fixing immediately. The lesser problem of elongation usually makes itself known by a reduction of power (because the cam timing is late).
However, on those vehicles using timing belts, a replacement schedule makes sense.