The "recommended distance they should be apart" is zero: both grounds should be bonded together at a common ground point.
In the United States, the National Electrical Code specifies that ALL electrical systems entering a building must be bonded together and grounded to an approved grounding conductor at the point of entry. This includes:
- Electrical power neutral conductor - Exterior electrical circuits for lighting, water pumps, etc. - Telephone lightning protector - Telephone shield and/or messenger, if any - Cable TV shield - External antennas (amateur radio, radio, TV, DBS, etc.)
It also includes fire and police alarm wiring systems if any. It might be a bit embarrassing if a fire alarm system started a fire.
There's an excellent article about all this at