From what I have read in this forum - it's about 8 feet. I'm sure a lot would depend on the type of camera, lens etc. but I believe the closer you get it to the human, the better
In my opinion, 8 ft won't work if the camera is two feet away. I think what you really want to have is a low angle - best recognition would be at head height or slightly below to see under the bill of a hat or cap. The farther away the camera is, the higher it can be. Of course you might need a longer lens to get a large enough image and that may require more light. As suggested previously, on the 2nd story is most likely not going to be effective.
Also see the test rig and what we recommend doing to test potential positions.
Summary:
1) Need enough effective pixel density on target.
2) Need a good angle for ID purposes.
3) Test all planned positions for effectiveness before doing a final commit.
In general, 2nd story eves are very poor positions for most cameras that you want to use to ID. Exception of course is if you can reduce the camera to target angle and zoom into a target further away from the camera so that you have a high effective pixel density.