Actually a bit over 500 yards in 20 seconds for the MK 14 on high speed.
http://www.subsim.com/radioroom/showthread.php?t=219562
Big sonar cheat when playing with realistic targeting - in real life every time you ping with active sonar, anyone in range with a hydrophone can hear it and get a bearing on you. The game didn't program that in, you can ping destroyers constantly and they never hear it. So to get an accurate range line up the bearing, hit the lower left switch to ping, then the lower right to send range and bearing to TDC. Even if it's way beyond the 5000 yard max range on the indicator, it will display in the message window and on the TDC.