There really is no one answer to this. It depends on you. The shortest I've ever heard of someone doing the self-study course was 6 months, and I believe he didn't have the distractions of a job and came from a military aviation background. Most people take 9-12 months, so the estimate given seems sensible. You will probably get through the early stuff faster than the later stuff, as fatigue sets in, so plan accordingly.
Ultimately, decide how much time you're prepared to spend on this (15 hours per week is about 2 hours per night, every night - are you going to study weekends too?), and stick to your plan. Be realistic! And allow for revision time, too.
Are Bristol still doing the 'transitional' (i.e. fixed-wing based) course, out of interest?