My dad was a Flight Engineer on Halifax IIIs. He joined the RAF as ground crew aged 20 in 1939. After serving in the UK during the Battle of Britain he was posted to a training squadron in Canada in 1941 where he eventually became restless and volunteered for aircrew in late 1943 and got through selection despite having poor eyesight! He joined his squadron in June 44 just post D-Day and flew his last and 36th operation in March 45. His Pilot was ex-Battle of Britain Fighter Command and was aged 27 but in my mind the absolute winner in the bravery stakes was their Rear Gunner who celebrated his 48th birthday in 1944. Regarding how long a tour of operations took I met another Flight Engineer at one of dad’s reunions who said he didn’t know anybody there as he had done his 30 ops in 6 weeks flat before being sent to India as an engineering officer and had never flown again. So anything from 10 months to 6 weeks!