Many quantitative, log-based studies of participation and contribution in CSCW and CMC systems measure the activity of users in terms of output, based on metrics like posts to forums, edits to Wikipedia articles, or commits to code repositories. In this paper, we estimate the amount of time users have spent contributing. Through an analysis of Wikipedia log data, we identify a pattern of punctuated bursts in editors’ activity that we refer to as edit sessions. Based on these edit sessions, we build a metric that approximates the labor hours of editors in the encyclopedia. Using this metric, we first compare labor-based analyses with output-based analyses, finding that the activity of many editors can appear quite differently based on the kind of metric used.