Average is 12-16 months for weekend-only flyers, judging by my old flying school. Caveat is that you stay on top of your ground exams, revise thoroughly between lessons and use at least a portion of your summer hols to do some intensive training. It also helps if you become a 'regular' at your flying club - you'll be surprised at the extra knowledge (and even free flying on occasion) you'll pick up by just being a known face. Do take any advice given to you by the non-instructor, club stalwarts with a large pinch of salt however. Beware the club know-it-all who says he's forgotten more about flying than the CFI has ever learnt - they can do more damage than good when it comes to giving you advice. And one last thing, it's not a good idea to train at a school that will give you a different instructor for every lesson. There's nothing wrong with swapping between a couple of instructors, but much more than that and your training will become muddled.
Regards, jez