To see how much time was billed to a project, you can open that project and look in the time tracking tab.  However, if you want to see everything that was billed in a particular period of time, you can run a billing report by going to Reports in the main menu, then click on the "Project Time Tracking" report menu.

On the report screen, you can filter by date range, project status, project type, etc.  Once you select your search criteria, click run report to see the results.