They never become available.
Username accounts are not deleted. For more information, please see http://en.support.wordpress.com/deleting-accounts/
1. You can go to My Account->Edit Profile and remove all data except the account name and an email address. Create a throwaway email address and change the account email to that throwaway address. This only leaves the username behind.
2. To unsubscribe from email sent to you from WordPress.com, go here and click on “Block all subscription emails from WordPress.com”:
3. To ensure the account is locked up tightly, visit this site, generate a password of 15-20 characters, and change the account password to it: