I could only have lessons on a Saturday, due to a combination of work and home commitments, and it took me about 18 months.
The weather very rarely cancelled a lesson outright, but frequently delayed solo circuits/XC flights by being outside of the club's minima for student solo (or my/my instructor's comfort zone for solo work). On more than one occasion my instructor used the privileges of his IR to get us away from the weather at base to some clearer weather elsewhere for a dual lesson.
I would say that the weather probably added about 3 months to my training time, but that said, I'm glad I experienced a variety of weather conditions before qualification!