GPG keys can have an encryption subkey, which is used only for encryption of messages. The encryption subkey can be revoked and replaced periodically. The old private key must be securely destroyed.
You can do this by choosing Actions/Rotate Encryption Subkey, clicking [Rotate Key], and then clicking [Destroy Backup]. The current encryption subkey will be revoked, the previous revoked encryption subkey will be deleted, and a new encryption subkey will be generated. Your key will be immediately reposted to the server.
You can change your encryption subkey once a week, with little risk of receiving mail you cannot decrypt. Your conversation partners will automatically refetch your key. It requires two key changes to permanently destroy an old key; the first one revokes the key, and the second deletes it. This means the minimum exposure is roughly the last two weeks - much better than "forever" if you don't rotate your key.
Your GPG directory is stored under your identity's home directory, shown in the chooser when you start up. Any backups of that directory, including Windows restore points, can retain old encryption subkeys and therefore increase the exposure period. It is up to you to mitigate that risk if you are serious about forward secrecy. Confidant Mail cannot do that for you.
If you use key rotation on an account that is shared across multiple machines, you will have to manually copy the GPG directory to the other machine(s) after doing the key rotation. There is no easy way for Confidant Mail to do that automatically without compromising security.
Any mail that is still on your hard drive is exposed if someone gets a copy of your machine, but rotating your key periodically does reduce the risk of a key compromise silently exposing all of your private communications.