6 seconds.
Ok, maybe a little more verbose.
If you fly without SCA (supercruise assist), power to you. When you've locked onto a target in supercruise, you will see (right side of the targeting circle in the main HUD) the distance and your ETA at your current speed. As SC speed is a function of throttle setting, local gravity gradient and your distance to target, that ETA will vary wildly as long as you're far from the target. As you approach, it will come down to more reasonable values - all you need to do now is to set your throttle to 75% ("middle of the blue") at or before your ETA reaches 6 seconds. Keep it there, and your ship will do the rest and bring you to a safe speed at a safe distance.
If you refer to the distance and speed indicstors on the left side of your radar, at your target display, that is useful for fine tuning your speed and getting an idea about how close you are to a safe drop. Those last "6 seconds" can stretch quite a bit. With a bit of experience, looking at that distance/speed indicator, you can get a feeling about what throttle setting you can get away with at which distance.
Those "6 seconds" aren't always safe, though. Sometimes, local gravity tricks you and you will still overshoot. Console yourself with the thought that an overshoot and a tight loop is faster than throttling to 6 seconds far out.