The terms and conditions say you have 1 year to activate. (Current Passholders will be able to renew their Pass outside of their allotted renewal window by paying their difference in full)
PASS EXPIRATION: For a new Pass purchase (not a renewal), a Pass is valid for applicable privileges for one (1) year from the date of use of the Pass for first entry into a Walt Disney World® Resort theme park. Pass privileges will not be available until such Pass is activated via park entry; provided, however, that park reservations, as available and subject to associated restrictions, may be made in anticipation of activation of a Pass so long as such Pass has not expired.
A Pass will expire if it is not activated within one (1) year from the date of purchase, and upon expiration, such expired Pass may no longer be activated or have park reservations scheduled for it, and any existing park reservations for such Pass will be cancelled. No refunds will be given. The original Pass may not be transferred, and the amount paid for the original Pass may not be redeemed for cash or be used for any other purpose other than the purchase of another Pass or ticket. However, the amount paid for an unactivated, expired Pass may be applied towards the purchase of a new Pass or ticket (subject to availability) at the then current price so long as the new Pass or ticket purchase price is equal to or greater than the amount paid for the original Pass. The then current terms, conditions and benefits associated with the newly purchased Pass or ticket will apply at that time.