From how far strobe lights are visible?
For cell towers the FAA wants them to be visible for pilots in time for them to avoid them.


  Pilots of aircraft travelling at 165 knots (190 mph/306kph) or less
  should be able to see obstruction lights in sufficient time to avoid
  the structure by at least 2,000 feet (610m) horizontally under all
  conditions of operation, provided the pilot is operating in accordance
  with FAR Part 91. Pilots operating between 165 knots (190 mph/303
  km/h) and 250 knots (288 mph/463 kph) should be able to see the
  obstruction lights unless the weather deteriorates to 3 statute miles
  (4.8 kilometers) visibility at night, during which time period 2,000
  candelas would be required to see the lights at 1.2 statute miles
  (1.9km). A higher intensity, with 3 statute miles (4.8 kilometers)
  visibility at night, could generate a residential annoyance factor. In
  addition, aircraft in these speed ranges can normally be expected to
  operate under instrument flight rules (IFR) at night when the
  visibility is 1 statute mile (1.6 kilometers).


http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2070%207460-1K.pdf (page 63)