Depends on the sea state and visibility. At night when the water is choppy and the wind is kicking up spray and whitecaps they might never see the wake at all, but in clear weather daylight with calm seas they can see the wakes from as far as 4000 yards. A Mark 14 set for high speed goes about 1500 yards per minute, so if you shoot from 1000 yards the target has about 40 seconds to detect and react. 500 yards gives him 20 seconds, time enough to say "EEEK!" but not much else. 2000 yards gives him one minute and 20 seconds, which is often enough. Attacking at night in choppy seas I have gotten a good percentage of hits with overlapping targets from 8000 yards with the fish set for low speed, but anything over 2500 yards in the daytime you need a lot of luck to actually hit anything except anchored ships.
When you think you're close enough, get closer.
