I did my PPL at FIS in April - May 2015. I started from scratch and just completed in four weeks. It really was pushing it though, and I would have been much less stressed if I had given myself six weeks, but my window was really limited. I took my last skills test on the afternoon before my flight home.
Like others, there are times when you cannot fly even once a day because of the weather. These days are infrequent, but you only need two or three with a tight two flight a day schedule to get behind. Similarly, one instructor became ill for a couple of days and another went home for a short break which left the school very short.
When I went to the manager / coordinator and explained that I was falling well behind in hours they were very helpful and worked hard at getting me up as much as possible. I am not sure how many instructors they have now as some of the ones there during my time transferred to the FTE outfit.
You will not be charged for hours you don't use, although you have to keep your flying account in credit. They do fly on Saturdays and can train on Sundays as well if necessary.
When I came home with my licence, I undertook further training to gain confidence in the different conditions. R/T discipline is also tighter over here, but that shouldn't be a problem if you have been flying in the UK and have already taken the exam (I took mine immediately on my return home).