2 Rounds of the scout's full movement when running, normally. Minimum of one round of hauling ass. So for a normal human, normal scouting distance would be somewhere around the 240' mark.
Reason for this is twofold - first, means the scout can move back to the party in a short amount of time if and when it's needed.
Second, the range penalty for whatever the scout is scouting to hear/spot the party will be at somewhere around -24. Enough such that the normal un-sneaky folks can stumble about without too much effort to be cautious and not automatically give their presence away.
Low lights and some form of communication (message, telepathic bond, familiar on point with the scout as a nannycam, etc) are very useful.