You have 3 years to complete the CPL and IR before the ATPL exams expire, but for a FI rating the requirement is to have demonstrated CPL level knowledge and the exams remain permanently valid. If you consider that the hours requirement to get a FI is higher with a PPL than a CPL, not completing the CPL might be a false economy. There is no way to keep the exams valid past the 3 year point for an IR.