That depends on the voice codec used i.e. G.711, G.729, G.723.1, G.726, etc. Cisco documentation specifies an additional 5% overhead be added for call signaling and control.
Visit http://www.btscomllc.com/technology.html and try out the Cisco VoIP Voice Port calculator. You have your choice of an online java calculator or you can download an MSExcel version.
These include the Cisco recommendation of 5% overhead for call signaling and control, so you can see how much each codec uses based on the voice codec selected.