For task/issue/project approval paths there is no way to set a due date. If you set a duration for a task approval in the setup area, this duration is applied to the task the approval is added to. If you have your process setup to mark a task complete-pending approval the duration isn't honored because the task is complete.
The reviewer is pinged to review, but there is no way to know what is due when. Approvals filter into their list in order of last updated, so if you tag someone to say this is Urgent it moves it to the bottom of the list.
