The sales and revenue estimates in the Extension and Extension Lite (as well as the Product Database) represent figures for a 30-day period. However, that is 30 days into the future.
Basically, what that means is that if the product continues to sell at that particular Best Seller Rank (BSR), then the estimates you see in the extension and/or database would be the item's estimated sales and revenue for the next 30 days.
If you want to read more about how estimates work and how they are generated, you can check Greg's blog on the science behind our estimates: AccuSales™. It is full of great information and interesting facts about them!