You'll need to check with the ground school as to whether they have a time limit. What usually happens is that you pay upfront (i.e. £1,500 to £2,000 ish) for the complete course which will include brush-up classroom courses as and when you sit the exams. Some schools (due to course material going out of date) may have a limit of two or three years from originally signing up with them.
Cheers
Whirls