If you're worried about the Pause Until action in SharePoint Designer as described in a SharePoint Workflow to send Email on a specific Date and Time, perhaps you might want to look at it's Visual Studio Counterpart DelayUntil.
If DelayUntil is too easy, or not enough of a difference from Pause Until, I suggest reading up on Stages in SharePoint 2013 Workflow in Visual Studio. Andrew Connell has a great write up of how things have changed in 2013 to allow better scale and performance without putting a burden on your main SharePoint Farm.
In addition, there's a very complex state machine Visual Studio example on MSDN samples from the same author, SharePoint 2013: Route workflows to states depending on actions and events. The example includes a delay.
For more on approval workflows in SharePoint, I recommend checking out the MSDN Code Samples for SharePoint. There are some great workflow examples from experienced SharePoint developers like Andrew Connell. And a few cover workflow.