There are rather too many variables to be able to give a simple average, principally: type of aircraft, definition of "mission" and geographical location.

1941 is a good starting point, as that's when the Air Ministry started to formalise the idea for tours of duty. Prior to that it was generally down to the judgement of individual Commanding Officers, and COs still had some discretion under the guidelines issued so things could vary.

Tours varied between the main units of the RAF: Fighter Command, Bomber Command and Coastal Command (there were also other areas such as Transport Command and Army Cooperation Command). In Fighter Command, tours were generally judged by flying time rather than number of missions; in 1941 the Air Council issued a general rule that "personnel should be relieved from operational flying after a maximum of 200 hours in one tour". After a tour, pilots would be rotated for six months, typically as an instructor in an OTU (Operational Training Unit), before starting another tour.

As sorties could be anything from low-level "Rhubarb" sweeps over enemy territory to long, tedious defensive patrols over convoys with a fairly low probability of encountering enemy aircraft, the contribution of flying time to the 200 hour total could be adjusted; generally, defensive sorties only ranked as 50% value when calculating the time.

Even within Fighter Command there were variations, with night fighters at times having shorter tours (100 hours), and fighter pilots in Air Command South East Asia having longer tours (300 hours); tours could also finish after 12 or 18 months if the flying hour totals weren't reached.

In Bomber Command, tours were generally by number of completed operations, typically 30 for a first tour, 20 for a second tour (again this could vary, the Pathfinder force did tours of 45 operations). A sortie was considered a completed operation if the target was successfully bombed, so aborted missions didn't count. Again, the types of mission varied from bombing heavily defended targets deep within Germany to dropping leaflets over occupied Europe, the latter counting as only 1/3 of an operation.

Very broadly, a typical fighter pilot might rack up 250 - 500 combat missions from 1941 to 1945; a bomber pilot, perhaps 50 - 100 if they survived (considering Bomber Command losses were averaging 5% at times, completing one tour, let alone more, was statistically unlikely).