Hello @Comms ,
If the survey is still active, you can extend it indefinitely by editing the expiration date (under the survey settings link in the top-right corner of the editing page) to be 6 months further in the future each time. If the survey has already closed, you won't be able to reopen it and will need to make a new one.
At this time, there isn't a way to re-publish the same survey page (and link) after it's closed, nor any other way to extend its longevity. I've tracked the known feature requests regarding these functionalities to your account. If and when the devs have an update regarding them, they'll notify you directly.