Usually you're evaluated after every engagement but it varies from manager to manager. The thing is to make sure you're getting feedback when you need it so can work on improving whatever it is by the end of a project. That'll help make sure you're entrusted with more responsibility on the next assignment.
At KPMG, evaluations are taken very seriously. Like any Big 6 firm its an up or out mentality at work. When they rank you at the end of the year its based primarily on your seniors evaluation. It also based on your knowledge of industry standards and the accounting practices of the area you're working in. Again the most important thing is to seek out feedback and work on improving areas where your deficient because most of the managers really want to see you improve and come back next year.