Recent grads or students who aren't currently taking courses have up to 18 months after their graduation/last course to use their card online. The key word being ONLINE. This is due to Active Directory (AD) access expiring after 18 months which is required for authentication. The funds on the account never expire and students can always refresh their AD access by going to my.nait.ca and clicking Sign Up. They can also use their card in person.
Staff is a bit different as staff use their AD access for more than students and as such their access is turned off immediately. Additionally, the current HR process pays out their one AT NAIT card balance as part of their settlement package. This means that staff usually have no funds in their one AT NAIT account after leaving NAIT.