The term 'stretch' is not strictly correct. The actual mechanism is wear on the pins and link holes. The practical limit is the 6 deg offset key, and is that because any more offset would result in too weak a key and potential for a sheared key and resulting catastrophic engine damage. The chain (double row) is plenty strong to handle LOTS more wear before weakening enough to be of concern. However, a worn chain will also cause abnormal wear to the sprockets. If the new chain does not fit into the bottoms of the teeth smugly, the sprockets must be changed. Also, if the sprocket teeth have any 'hook' or concave wear on the driving side tooth, they must be replaced or rapid chain wear/failure can occur.