About 6 months......
Well, the guy concerned was also less than 16 years old - joined RAF in early days of WW2, lied about his age, adding 4 years on - "Honest, Sir, my birth certificate got posted to you." He was 15 yrs & 6 days old when he flew in his first aircraft, a Tiger Moth. Went to Canada for his training & was awarded his wings when he was 15 years, 4 months & 9 days old!! Soon afterwards, as a Sergeant pilot, he was the captain of twin-engined Whitney bombers, flying operational raids.
The guy's name was Tom Dobney - only got caught out 'cos his mum (who thought he was helping the war effort by training as an air cadet) saw a photo of him chatting with King George VI, who had been touring air bases in East Anglia. Poor lad was discharged, aged 15!!!
He kept trying - got thrown out of the Fleet Air Arm, Air Transport Auxiliary & again, the RAF. Still, he finally got back into the RAF!