Yes, sorry about sending you off in the wrong direction. I was too quick in my original answer..
If you look at the Create Multiple default form you have a field there called "Account Lifetime" which is what you need.
Put the Account Expiration to "Do not expire" and input something like 8 hours in the "Account Lifetime" field.
Editied: If you want to have this field in another form, the name of it is "expire_postlogin". By default this field is not part of the create_single user form.