The following is summarized from the CIS athletes guide which can be found in full HERE.
At USport (formerly CIS) institutions you are eligible to compete in your sport for FIVE years. There is no running clock, and you may take years off between athletic seasons if you wish. Note: in the sport of football, student-athletes must be 24 years of age or younger as of August 31 of the year of their participation.
What is considered a year of eligibility?
Depending on the sport you play and the schedule of your team, you can be charged with a year of eligibility when your name appears on the playing roster (or equivalent) of as few as one or as many as three competitions; each institution’s Athletic Department will have clarification of their rule.
for National Team, professional, foreign, and other non-university sport participation, each institutions Athletic Department will have outlined policy’s. These activities can result in the loss of eligibility or the requirement to sit-out of USport competition for a predetermined period.
Athletes who have completed their eligibility to participate in another post-secondary jurisdiction, such as the CCAA or NCAA, are ineligible for USport competition.
For information about transferring between leagues please see this link