How long for skin to grow back over a big cut?

[Hampton Bay]

My yearling filly cut her leg down by the coronet band last Saturday. The cut is about 4" long, down to the tendon near the heel. It is healing pretty well without getting infected. There is about an inch gap between the pieces of skin, and that is mostly filled in with healthy-looking tissue. But how long until I start to see skin growing back on the less severe part of the cut?

I can call my vet and ask, but somehow I always forget during business hours.