I have not used it but I belive "Shutdown Now" has the functionality you want. It is available
here. It is not specifically designed for faxes. However, I believe there are a couple ways one could do it with that program. One could have it shut down your machine after a certain period of inactivity. One could wrap a script around your fax receive software to shutdown after a certain period. I am sure you could come up with something with that program or at least I don't have a better idea for you. It is shareware so you can give it a try and see if it works for you before you buy it.