Its really up to you however we recommend removing people if they haven't opened an email in say 6 months but it depends on the ISP as well sometimes i.e. in case of gmail, you would want to clean them out sooner so as to not lose engagement and there by lose your inboxing.

This feature coupled with our new workflow engine allows you to setup an automated re-engagement campaign to target people say after 5 months of not opening with a series of emails to get them to open.