The "Active Collaborators" information, available for owners on the "Collaborators" tab, can be confusing and inexact. If a collaborator is logged in and clicking around, we start tracking, but we don't log it until a "deed" is logged. (A "deed" is one of the actions that show up in the "recent activity" list.) If you click for 8 hours and halfway through transcribe or leave a note, it will register the whole 8 hours.
If you find those statistics to be too inexact, you may choose to derive an estimate of how much time a single page takes to transcribe and multiply "pages transcribed" by that number.