90 minutes is the norm on BA for J (and 2 hours for Y); you can check on ba.com under MMB to see the actual checkin limit for your ticket. BA is rather inefficient in FRA, I'm afraid, not one of their better airports. The 'short' tags really do seem to work, getting priority over other luggage, and I've never had a short tag miss a connection in dozens of attempts -- while I have had bags lost at LHR for several days more than once, but they were F tagged and not short tagged; F tags seem to mean "steal me first" at LHR.