You can use our Timelines feature to track how many days worth of work you currently have in progress and, based on a three week moving average, how long it will likely take you to iterate through your backlog.

Sprint.ly overlays a dotted line with a black label that will tell you roughly how many days worth of work you have currently in progress:

In addition to this, Sprint.ly also overlays dotted lines every seven (7) days going down your backlog in the Timelines view.