Puttygen default passphrase

WinSCP and Putty use, and Puttygen generates, SSH keys, and SSH keys do not expire. (Unlike for example PGP signed-keys and X.509/SSL/TLS certificates for keys.) Puttygen by default puts the generation date (yyyymmdd) in the comment field of the key, and if you don't modify this, you can later use it to manually decide whether a key is 'too old' and should be replaced or deleted. Of course, usually the OS/filesystem provides a timestamp on the file (containing the key) anyway.

But note the passphrase is not the actual key, it is used only locally to encrypt the privatekey which is randomly generated (that's the bit when you must move the mouse around). If you want to change the passphrase for any reason, including the passage of time, you can Load the .ppk file into Puttygen, retype the KeyPassphrase and ConfirmPassphrase fields, and Save. The actual key is unchanged and will continue to work with a server where you (or an admin) have previously configured the public part of your key as an 'authorized' key.