Usually when a fish is eating well and healthy, their fins will grow back over night if the tear is not too bad. Although for some fish it might take longer for example; my lionfish I used to have took a couple months for his spines to grow back to normal. Just the other day, my yellow tang got nipped by my picasso, he had a pretty good tear in his dorsal fin, it only took him a day and he's good to go.
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)