As a free user if you do not receive a fax from a unique (different from the number from which you previously received a fax) every 7 days, you lose it.  You can get it back at any time by upgrading to a paid account as long as another user has not already paid for that number. You can check if your expired fax number is still available by logging into your account and clicking on the "Upgrade" button.  If your number is available it will be displayed above the pricing options.  If no numbers are displayed, none of your past numbers are still available.  

The 7 days is time based not date based, so if you received a fax last Thursday at 10am and then sent a fax to yourself at 2pm this Thursday, your fax would have expired at 10am this Thursday.  Expired numbers stay out of circulation for a while but eventually get assigned to a new free user. 
 

If a prior free user of your expired number upgraded to a paid account then that number would be assigned to that paid user even if your 7 days has not elapsed.
 

Once you log into your account, your current fax number is always located in the upper right hand corner.