I've heard that with favourable conditions and commitment, under 2 weeks has been achieved by some.
In UK, they say it takes months because of weather, lack of aircraft etc. but, on pay-as-you-go, the longer you take the more money they make !
If you find a 3 week course worrying, just ask OBA to quote for 5 or 6 weeks. As far I know, OBA will only charge you for extra accommodation time which was cheap and quite acceptable (modern single storey houses just outside the airport site). You could pay extra to guarantee your own en-suite room.