I started in September last year and finished last month, so that's around 8 months studying part-time with an awful lot of distractions (job, kid, house purchase/move etc...). I guess I averaged around 2 hours a day.
I suppose you can study 8 hours a day for short periods (i.e. a couple of weeks before the exams) but realistically you're probably not going to get more than 4 hours of productive studying done per day. I wouldn't choke on my cornflakes in suprise if others had completed in 4-6 months.
Everyone is different, but you already have a strong background so I would have thought that 6 months was perfectly achievable.
Agree completely with Drakestream and Mad_jock, keeping motivated is massively important so if you can fly a little take the opportunity and the volume of the information is the biggest hurdle.
Good luck.