36 months
You get 36 months after your last exam pass to obtain your CPL and IR before they exam credit expires. You get 7 years to get your mult-time to obtain your ATPL before those few exams expire. There are ways to interchange fixed wing ratings to help yourself, if you have a FW ticket.
Get yourself a copy of LASORS 2006 - It really is a worthwhile £13.50 to spend and it answers all.
HH