The license is forever but there's a small annual fee if you want to receive updates. You don't need to pay the update fee until you actually want to update and you could continue running your forum using the original version you download without ever updating if you don't want to (serious security updates are free).
Also they've previously stated that your license for XF 1 will remain valid for XF 2 when it comes out so there aren't really any end of life issues like you have with other forums.