You'd want to enable the units of measure feature in your QuickBooks. This way, you can track and display specific information (such as the quantities, prices, rates, days, and costs are based on) about your list of products and services. I can guide you on how to do it.
Here's how to enable it:
Go to the Edit menu.
From the Items & Inventory menu, choose the the Company Preferences tab.
In the Unit of Measure section, select Enable.
Choose Multiple U/M per item.
Once completed, you can now open your item list and assign specific unit to each of your products and services. Here's how:
Open your item list.
Right-click on the product or service item you want to use.
From the Unit of Measure drop-down, click Add New.
Select the unit of measure you want to use.
Follow the onscreen instructions to complete the process.
Any unit you assign will automatically show when creating an estimates and other sales forms.
Units of measure isn't available in the Mac version. There is no option for multiplication except for the quantity x price. For that reason, to use the built in invoices functionality, you'll have to incorporate the # of days in the description or another field and do the multiplication manually in the price column (ie... enter the line item and press * when in the price column and then let the calculator widget do the multiplication to the price).
I know its not exactly what you want but what you want isn't there.