As long as it's just on the fins they will grow back, if their is damage to the body below the fin then they may not.
With in 2-3 days if their healthy you should see it starting to grow back, your get a thin clear edge to the area where it's growing back.
But now you gotta watch that no fungus set's in the the wound, if your doing weekly water changes and checking your water quality then you should be ok, but having some anti-bacterial and anti-fungal treatment's is very handy with new fish.
If you notice any redness to the area then use some treatments.
Helter