You will always be asked to leave the feedback after each trade. It’s best to do it right after the completion of the trade. But if for some reason it cannot be done, you have 14 days to do it. The countdown on top of the feedback page will remind you about the time left.
If after 14 days you don’t leave any feedback, the system will not consider this choice and it will not be reflected in user’s Public Profile in any form: Rating or Statistics. Therefore, it will not affect the other user’s overall reputation score negatively. It will be shown only in user’s Personal Profile for general information.