Well, others may have more detailed info, but if he was sent for initial training immediately after enlisting on say 1 April, then I would think that he could have been sent for AG training in perhaps June, since initial training was probably only 6-8 weeks. If he was an AG and not a WAG, then he'd only have to take the gunners' course, which may have been as short as four weeks, taking him to maybe mid- or late July. (If you could find out the date of his promotion to sgt or the date he received his AG flying badge that would tell you when he completed his AG training.) Hopefully he then attended an operational training course at what was later called an OTU but which in 1940 may have been called a reserve squadron. If this was just another four weeks or so, then I think it may have indeed been possible to join up in April and join 78 Sqn in 5-6 months, if everything clicked, e.g., if each course started just after the previous one ended and ran smoothly, if he lost no training time due to injury or illness, etc.