Very friendly, Shoe!
Between a possible advantage over your opposition and friendlyness towards the blokes playing with me, I consider to concede within 3 feet if I can judge it as an easy straight forward putt. Not when I know it's a cracking side-sloping downhill putt!
As Shoe indicated, the concede is a very personal instrument, and a tactical one, too. You're allowed to use psycho-tricks in A/S and single matches!
Therefore, one thing I detest very much is the prior agreement to concede within a certain range. It is at the sole discretion of the conceding player!