The Product Duration feature enables you to use an additional field when attaching products to deals.
You can enable Product Duration for your products by going to Tools and apps > Products.
Once it's enabled, you will be able to choose a product duration when linking a product to a deal.
This is useful in the context of both time and amount related products, such as hoteling or warehouse services. For example, if the product is a hotel room, Product duration could be used to indicate the number of nights a potential client might occupy that room. So the price is per night, and the duration is the length of the stay, defining the deal value (Price x Duration = Value.)