Is this ETOPS? If not, you will always have a diversion within about 330nm of the return track. You can set off therefore, back to departure ?IF YOU MUST? and keep it under review once you know the fuel burn and winds at 10k. If it is ETOPS, your PLOG should have a fuel requirement for up to 120/180 mins at 10000 feet from which you can extrapolate an intelligent guess. Personally, I would, however, have thought that after a pressurisation failure you would want to land fairly soon?