The best time is obviously before you have concerns.
We suggest you undertake Critical Point Reviews (CPRs) from the very inception of the initiative until completion. For example:
CPR-0 Strategic Assessment
CPR-1 Review the Justification
CPR-2 Delivery Strategy
CPR-3 Investment Decision
CPR-4 Readiness for Service
CPR-5 Operational Review & Benefits
We also find if there is a change in leadership of the program or project, a review saves considerable headache!