Our users report that three weeks is plenty of time to register students for exams. We recommend having students register for three weeks at your standard exam fee and then having an additional week of registration with with a late fee. Schools that use a late fee report that students will rush to beat the late fee date. We do have schools that run their registration period for months and others that register students for just a couple of weeks. In general, students will wait until the last minute to register, so having a long registration period is generally not beneficial.
We do not recommend less than two weeks for the registration period as it is important to allow time for the communications about the registration to reach all students and provide time for exam payments to be made.