Any time. I've been there and it's a sucky situation. I understand it's tempting to give them a piece of your mind, but I've found that taking the high road helps.
The problem is you giving honest and brutal feedback feels good in the moment but you never know when any connections you have will come in useful in the future, and if you will burn those connections with your feedback.
You don't stand to gain much by being honest on your way out, but there's a possibility of losing out if you are. So take the high road.