It's been basically 8 for a while. Not sure what the official rule is, but I usually do the math for some reason, and teams have generally been doing 8.
There is no set rule where it has to be, only that it has to be behind the LOS. (duh) Teams calculate snap speed and projection of the kick to give the kicker the best chance to beat the rush while staying as close as possible. With kickers being stronger and more accurate, the extra yard gives them that extra split second to get the kick above the rush.