The ATPL theory practically never expires - unless you quit flying helicopters for more than 7 years. See Part FCL.025:
(c) Validity period
(2) The completion of the airline transport pilot licence (ATPL) theoretical knowledge examinations will remain valid for the issue of an ATPL for a period of 7 years from the last validity date of:
(i) an IR entered in the licence; or
(ii) in the case of helicopters, a helicopter’s type rating entered in that licence.
If you don't manage to get an initial instrument rating within 36 months of passing the ATPL/IR theory, then you have to do the IR theory again. Similarly, if you let your IR rating expire for more than 7 years, you have to do the IR theory again as well.