Well I got out (as FTRS aircrew) in 2005 with only about 4 weeks notice after I had a solid job offer in civvy street. All I had to do was fill in a single A4 sheet sized form and Innsworth agreed my release date by return of post. It was all very informal, though of course it would all depend on how your desk officer viewed your usefulness to the Service at the time. In my case, my contract had about 14 months to run but my particular role was facing cutbacks (one out of three squadrons being disbanded).