PCB's with Connectors
Some USB connectors provide drawings to help determine the overhang beyond the edge of the board. 

For example Molex 56579-0519. The second page shows a useful diagram:


This shows the distance between the plug and the end of the socket (1.5mm).

I believe Ethernet sockets have similar diagrams too.

Parameters still available are the thickness of the case walls, distance of PCB from the case, and whether their are cutouts for plugs.

I don't think there are general rules, but as shown by this example, manufacturers datasheet diagrams usually show clearance dimensions.