Re-Gaining lost weight is certainly almost always faster. But I had a lover who would torment me with re-gains and with very rapid new gains. But -there was one consistent fact with her. She had yo-yo'd at a 25 to 30 pound rate starting at her early teens. So she could gain 3 pounds a day for over a week -even new pounds. Her metabolism was shot. So in at least one person I've known on an intiment level-the answer is 3 to 4 days.
I just remembered something that I haven't thought about in 25 years. I played in a band job on trombone and there were two brothers playing tenor sax. They were big guys-6 foot and high 200's if not over the 300 mark. During a break one of them said that they had gained 12 and 14 pounds over the 4 day Thanksgiving weekend. I don't know if it was re-gain or a new record for each. I didn't ask because they were guys. They were good tenor players though-that much I know.
14 years