The IP is permanent until the droplet is destroyed.

One ip is assigned with one droplet & its static one. If you want to change ip then you will have to destroy the droplet & create new one.

@LukeForrest

The IP is indeed a Static IP until you destroy the Droplet, though to build upon what others have stated, DigitalOcean tends to attempt to reassign the same IP to your next Droplet if it's created soon after the first is destroyed.

This doesn't always happen, so you shouldn't rely on the same IP being assigned after destroying one and creating a new one, though there is a slight chance of getting the same IP again. It all depends on timing.