You only evaluate and end the relationship when all the learning objectives have been met. You are evaluating in order to assess whether you have achieved what you set out to achieve in terms of meeting the original development need. By the very nature of going through the process you will be evaluating against your objectives continually, as you progress.
Additionally you will want to find out how you did as a coach. To do this you will probably employ some of the review techniques that you encouraged your coachee to use. Tools for evaluation can be found in Part 3.
You and the coachee may choose to celebrate the end of the relationship.