The WSOp Generic course takes six months to complete, at the end of which streaming takes place.
There is no guarantee that you would be streamed as a rotary crewman but, to date, 75% of students have got their first choice.
Time between application and starting is largely in the hands of the recruiters, adminners, and bean counters.
Are you aware that 55(R) Sqn at Cranwell have a 'wannabees' day on the last thursday of each month. Worth giving them a call to get on that. It will answer a lot of your questions, and you'll have the opportunity to meet current students who'll be able to give their points of view.