I am working on a workflow setup to control our Quoting Process and trying to figure a way that the state named "Quote Pending PO" could "expire after a certain time... any ideas?

Right now the workflow is set up with 2 transitions coming out of the "Quote Pending PO" state. One of them being "PO Received", and the other being "Revise Quote". The hope would be to after a certain time frame (e.g 7 days) the workflow would automatically use the "Revise Quote" transition out of the "Quote Pending PO" state.