My betta fish has a tare in its tail fin?

yesterday i had bought a betta fish and well his water looked really nasty so i changed the water.. well he kinda pulled a ninja move and he went down the drain. well thank god the grate in the drain had caught him and well i got him out real quick and put him in the water.. he is perfectly ok and is doing good but now he has a slight tear in his tail fin and i was just wanting to know how long it will take to heal / grow back