Within the association form of the approval workflow you are able to set the “Due Date for All tasks”. This setting specifies the date by which all tasks are due.
What if you want tasks to become due on a specific time on a specific day? In the user interface of the association form you are not able to specify a time value.
Although it is nowhere documented, it is possible.
For example, if you want to terminate all tasks on the 18th of June at 13h30 you can set the following value in the textbox:
Use the following structure to set a time value:
I know, it’s not really user friendly, but it works. You can probably use SharePoint Designer to tweak the association form to implement some extra logic.
One of the best references online about approval workflows in SharePoint 2010: All about approval workflows