It depends on the speed at which the horse approachs the jump. According to d'Endrody Give Your Horse a Chance at 4'6" Your take off would be between 5'3" (at 350 yd/min) and 6'5" (at 492 yds/min) and your landing would would be between 6'3" and 7'11"... so that's 11'6" to 14'4". There is also a foot note that says that with a single fence (as opposed to one in a combination) the distance can be increased by 25%. He then goes on to talk about the effect of oxers of various heights and of the position of the ground line.
The variability in the answer is because once the horse has taken off the flight is ballistic (y = Voy t - .5 g t^2 for the vertical component and x=Vox*t for the horizontal component). In other words, a horse now has vertical and horizontal speed, no longer just horizontal speed.
Not knowing your age or level of education makes helping you find an answer difficult. I will suggest you look up ballistic flight and use your fence height combined with a series of canter speeds (Vo) such as 250 meters per minute, 350 meters per minute, 450 meters per minute and 550 meters per minute (all common speeds found when jumping) and you will see that there is no one set distance. Of course you can not just plug and chug these numbers, be careful of the units!