It may take up to a week or longer. I suggest uping your dose to 2 teaspoons per gallon, add the new salt in slowly so you don't shock your poor boy.
Also, don't know if you know, but don't use any salts for more than 14 days (although at higher dosage like 3 tsp/gal I don't go over 10 days). You should see some improvement towards the end of the week but I would go the full 14 days if you aren't seeing much improvement.
Then if he's still not doing well and the black is still there, give him about 4-5 days break from the salt (although the longer the better) and then you can do another 14 day period of dosing.
And you won't see new fin growth until the rot is gone or if that part is not infected. To heal them up I recommend any sorts of Stress Coat because it's got aloe in it so it will start to heal them up in no time
well that's a lie lol, it took over a month for my boy in my avatar to regrow a portion he bit off. He just now grew it all back!