Congratulations on your new chickens. You are asking a question which has no definitive answer. The age of your hens and according to you the poor quality of food the previous owner used make if very difficult to determine an exact answer.
As mentioned above they have been fed a poor diet. Each bird will be an individual case. The answer will be when they return to good health and their egg production system returns to some kind of normalcy. I wouldn’t expect all birds to return at the same time, but would assume between a month or two, maybe longer.
Good luck
