While I never dared to overstretch the expiration time by that much, I do use Tetenal E6 working solution for one or two rolls more than specified and also sometimes for three weeks instead of the recommended two - with no visible differences in the end result compared to fresh solution. I do understand the tight limits stated in the manual, though. Just imagine someone ruins really important shots (think: wedding shots, newborn shots, ...) with developer that is used within stated limits but still doesn't work: APUG would be full of "Tetenal ruined my perfect $IMPORTANT shots" threads ...