Compact privileges expire on the same date of an individual's home state license that was in effect on the date the individual purchased the compact privilege. There are no prorations, therefore the full fee will always be charged. The date shown on your dashboard at the time of purchase will be the date the compact privilege will expire.
Therefore, in some cases, it may make sense to first renew the home state license before purchasing compact privileges and waiting approximately a week to make sure the PT Compact database has received the updated expiration date from your home state.