What do you mean by "License Duration"?
It actually means "Update Duration". So, let's say you choose a "3 Months" license. Even after 1 month, you can keep using the add-on (it's for you forever). You only won't receive add-on's updates after your license is expired.
Should I pay to remove the copyright from the footer?
No. You can remove it from add-on's options page. There is a checkbox for it and is free of charge.