Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for determining duration performance of websites. In one aspect, a method includes, for each of a plurality of websites, obtaining duration measurements of user device session visits to resources of the website, obtaining data describing a plurality of categories to which the website belongs; determining for each of the plurality of categories to which the website belongs, a category duration score based on the duration measurements, each category durations score being proportional to durations of time from the duration measurements; and determining from one or more of the category duration scores, a duration performance score for the website, the duration performance score proportional to the one or more category duration scores from which the duration performance score is determined.