What is the minimum required range for a timely warning?

When flying at or below 250 kt, a range of 2 nm (3704m) forward and 1 nm to the side and behind will give the pilots a warning at least 15 seconds before closest convergence.
Any range beyond that may be useful for tactical purposes but it adds very little to safety.

Calculation:

1. Worst case below 10 000 ft is normally two aircraft converging at 250 kt each -> 500 kt closing speed -> 257 m/s.
2. For a 15 second warning -> 3858 m -> 2.08 nm
3. The profile of a glider when seen from straight ahead at 2 nm distance is about as thick as a human hair held at arm’s length; almost impossible to see...