Typically evaluations are conducted at the end of a project. However, to gain the most from the process of evaluation, and more importantly to make changes that keep your project on course, it is useful to plan a number of regular periodic evaluations. They will help you see what is working and what is not, and help you improve your project while it is ongoing.
Periodic evaluations are best done when you reach certain milestones of the project – when certain stages are completed or when important decisions are to be made.
Further resources on project evaluation