I learned to fly over a nine month period at Welshpool, my local airfield, which specialises in three week courses. I well remember the long faces of people who'd come for three weeks and spent much if not most of it grounded due to the weather. I also remember the stressed out looks of others who'd done most of it, but were staying up nights studying for the ground exams, and were so overworked they'd stopped enjoying the course at all. There were a few who finished in the three weeks, others who got the bulk of it done and came back later, and quite a lot who either had bad weather or needed much more than the minimum number of hours, who came back, or went elsewhere, or gave up. It can be done, but not often, isn't usually enjoyable, and probably isn't a good idea for most people.