As Sycamore has stated, it depends on the drop tank fit. From Chivenor an FGA9 would normally have 2 x 230gal drop tanks whereas an F6 would have had 2 x 100 gal tanks. Fitting of extra tanks was quite involvedand time-consuming . It also required airborne flow checks so it is unlikely that this would have been done for a short notice launch from Chivenor. From practical experience (Gibraltar to Brawdy) with 2 x 230s plus 2 x 100s a sortie length of about 2¾ hours was just possible landing with minimums. This was flying at Range speed of 0.8M. You could probably add an hour to this if flying at Endurance speed. I regret that I no longer have my Hunter FRCs (Flight Reference Cards) to give you more accurate figures but I am sure that someone out there will have and thus be able to give you a definitive answer.