An engagement time computer system for serving online content based on user engagement duration includes an output device, at least one memory, and one or more processors. The processors are configured to determine a historical engagement time of an online user and to identify a plurality of online content items. Each online content item of the plurality of online content items has a target engagement time. The processors are further configured to receive a request for a content item. The request is associated with the online user. The processors are also configured to select a first online content item from the plurality of online content items based at least in part on the historical engagement time of the online user and the target engagement time of the first online content item and to transmit the first online content item for display to the online user.