The sales and revenue estimates in Extension (as well as the Product Database within Jungle Scout) 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 learn more about how estimates work and how they are generated, make sure you visit our AccuSales™ page. It is full of great information and interesting facts about them!