@i920098 wrote:
but if the condition is dat: specify an expiration time that takes effect after the "last login" of a guest account.
@kdisc98 was offering other fields that you can use for account expiration, of which there are a few. His/her original suggestion of using the field, "expire_after" is what you want. Go to your existing self-registration, open the Register Page -> Form and edit field for "expire_after". The default initial value here is 24 for 24 hours. Modify this to 8 or whatever you desire. Now, any users that register on that self-registration page will receive an account valid for 8 hours after the time of registration.