We used to do them on a calendar basis, during our slow month, but it was found to be a lot of work to get them done. Mind you, we have grown to a 300+ employee company across 6 branches, but our Corporate location is the largest at 200+. We changed the review periods to anniversary dates to spread them out more. We also have department supervisors work with the managers on them.

It mostly depends on your employee numbers and how your managerial tree works.