When sustainability is calculated, the Duration of Use value is used in conjunction with the Built to last value you specify in the Manufacturing section.
To get a more accurate picture of the impact of a design, the Duration of Use value is divided by the Built to last value. The result is rounded to the next whole number.
For example, if a product is built to last four years and its duration of use is 10 years, the calculation is 10 / 4 = 2.5, rounded up to 3. This means that to have the use of the product for 10 years, you will need a quantity of three. So the net impact on all environment results is three times that of a single product.
Conversely, if a product is built to last for 10 years and it's duration of use is one year, the calculation is 1 / 10 = .01, rounded up to 1. This means you will only need a quantity of one to satisfy the duration of use.
Duration of Use and Built to last have been added to reports and the Custom Column choices for Assembly Visualization.