Depends on how bad
Jai had a right auto-feast once night about 8 weeks ago and he's almost back to normal but you can see where it was because the membranes are thinner, a bit spotty and a bit tatty, but if I had before and after pics you'd be impressed with his progress.

auto-feast!!!!! thats one way of putting it.
sounds like your treating him well to get a good growth out of him in a short period of time, some take forever to heal and growth to start showing. Remeber that the colours of the betta is a combination of many, the base colour is black on all betta (except when there is a genetic flaw,which produce mutated colours) so what you see as darker is actually the black with his normal colouring pigment starting to develope again on top, he should get lighter again the more the regrowth continues