How do I make an activation link on registration expire in 15 minutes?

I have user registration and forgot password mails sent to the user.

I want 5 minute expiration for forgot password and user registration. Now the forgot password will expire after 5 minutes but new user registration link is not expiring after 5 minutes.

I have set the following changes in settings.php.

But it is not working for first time activation link.

$conf['user_password_reset_timeout'] = '259200';

How do I make an activation link on registration expire in 15 minutes?