There's three options...
1) The length can be permanent where it never expires.
2) Or it can expire after a period of time.
3) Or it can expire after a period of time with recurring payments.
Option 1, self explanatory.
Option 2, after the upgrade expires they are removed from the group and they have to purchase the upgrade again for that access to be restored.
Option 3, once the upgrade expires, PayPal will automatically charge them. When they purchase the upgrade in the first place, they will be told clearly that they are signing up to a recurring payment.