The exams remain valid for 3 years for issue of an IR from the date that you pass the last subject. Holding a CPL does not affect the validity of the exams.
FCL.025 Theoretical knowledge examinations for the issue of licences and ratings
(c) Validity period
(1) The successful completion of the theoretical knowledge examinations will be valid:
(i) for the issue of a light aircraft pilot licence or a private pilot licence, for a period of 24 months;
(ii) for the issue of a commercial pilot licence, instrument rating (IR) or en route instrument rating (EIR), for a period of 36 months;
(iii) the periods in (i) and (ii) shall be counted from the day when the pilot successfully completes the theoretical knowledge examination, in accordance with (b)(2).