Workflow Approval Timeout option

Dear  All,  Greetings !!1

Our apps version is R12.1.3. - We have workflow  for standard  Purchase Order Approval.  Most of the time few  approval members   delaying their  approval.  Due to this  sending PO to supplier getting delayed.

To avoid this - we need if  a approver is not approving the PO within  3 hours ( or say N Hours) - it should  skip current approve and be passed to next approver automatically.

Kindly advice us to  achieve this requirement.

Regards