If I follow the link Forgotten your password? and get an email inviting me to reset the password for my EPO account, how soon do I have to react?

The email will invite you to select a button (or enter a code), and you have five minutes to do so. If you don't react within five minutes, you can go back and again follow the link Forgotten your password? The system will send you a new password-reset email containing another button (or code) valid for five minutes.