As said, the hours are for life. But as Whopity says if you did any ground exams you'll have to have completed all the ground exams within 18 months of the first exam.
Maybe someone can confirm whether it's 18 months after the first PASS or 18 months after the first ATTEMPT?
In anycase, if you haven't done any exams, then you may just have to go up a couple of times for 'revision' in order to continue training, depending on how far you are in the course.