Salesforce allows you to export data manually once every 7 days (for weekly export) or 29 days (for monthly export). 

 

We can also export data automatically at weekly or monthly intervals. Weekly exports are available in Enterprise, Performance, and Unlimited Editions. In Professional Edition and Developer Edition, you can generate backup files only every 29 days, or automatically at monthly intervals only.

So, check the previous exported date and time. After a week or month based on the edition, you can use the service again.