How secure is the passphrase?

 Hey Allan & Chris,

Thanks for the help with securing the server! :)

I'm curious, starting another round of new SSH keys and wondering how much more secure is a passphrase enabled 4096 key than a non-passphrase 4096 key? I know that obviously it is more secure, but how much? Do you happen to have some statistics? If we assume that the only instance of my private key is a personally owned computer, how much security am I actually getting from a passphrase? Obviously if my computer gets compromised / stolen it's time to switch out ssh keys whether or not they are passphrase enabled, but...

Sorry for rambling, keep up the great work!

"R"