The limit is 7 years from last expiry. The following is the extract from Part FCL.635 IR
(c) Renewal. If an IR has expired, in order to renew their privileges applicants shall:
(1) go through refresher training at an ATO to reach the level of proficiency needed to pass the instrument element of the skill test in accordance with Appendix 9 to this Part; and
(2) complete a proficiency check in accordance with Appendix 9 to this Part, in the relevant aircraft category.
(d) If the IR has not been revalidated or renewed within the preceding 7 years, the holder will be required to pass again the IR theoretical knowledge examination and skill test.
The skill test can only be done by an IRE not an IRRE
Hope its of some help