If it's the light strip then it's 80 inches. Just the extension is 40 inches. Important note : there are 2 different light strips and 2 different extensions and they are not interchangeable! Make sure to look at the end of your light strip at what type of connector it takes. I know that mine, for example, uses the larger connector. A good rule of thumb is the larger connector one comes in the larger box. The smaller one, is it in the small box.
See photo below showing both sizes next to each other for your reference