Forgot the timeline part- Its extremely hard to eyeball a horse. Especially if you see them everyday. Use a weight tape and keep a calendar in the barn to to track. 3 weeks before you see a 15lb or more weight increase on average.
Of coarse extremely thin will improve much quicker, this is for a horse needing 50-175 lbs. And some amount of exercise is good. Alot of time its not weight but muscle atrophy.