Dwell time as a user behavior has been found in previous studies to be an unreliable predictor of document usefulness, with contextual factors such as the user's task needing to be considered in its interpretation. Task stage has been shown to influence search behaviors including usefulness judgments, as has task type. This paper reports on an investigation of how task stage and task type may help predict usefulness from the time that users spend on retrieved documents, over the course of several information seeking episodes. A 3-stage controlled experiment was conducted with 24 participants, each coming 3 times to work on 3 sub-tasks of a general task, couched either as "parallel" or "dependent" task type. The full task was to write a report on the general topic, with interim documents produced for each sub-task. Results show that task stage can help in inferring document usefulness from decision time, especially in the parallel task. The findings can be used to increase accuracy in predicting document usefulness and accordingly in personalizing search for multi-session tasks.