I've cap-cut several fringe players and then gotten them right back. I've cap-cut several good players and they still hate me, after 2 seasons! It seems players whose default rating is better get mad quicker and stay mad longer. Players who should be glad to play anywhere, for anybody, can be cut nearly at will. If you trade a guy, the happiness goes to "Livid" and then stays there.

I traded a player during the 1987 season and entering the 1989 season....he is still livid with me.

Nothing else to add...just an example. If he were to become a free agent....He wouldn't sign with me again.

Ive seen players who are still livid 5+ years after getting traded, so I think the answer, if its even possible, is a longggggggg time.

Getting back to neutral assumes you re-sign them and play them every single game
I got from Livid to Very Unhappy pretty quick (4 games? 6?) with default rating 54ish players, but over the course of the whole season it didn't go past that through ~10 games

Players move back towards happy faster if you're winning.