In addition to what Acera said, here's a guide on measuring the LOP someone needs, and how to check a stock to see if it'll be the right length.
How To: Measure YOUR Length of Pull
Using that guide, I need 13.75 inches of LOP, which explains why most factory stocks seem just a hair short to me.
Also, fwiw, the longest factory stock I've found is a MagPul ACS with their enhanced butt-pad, which should come in around 15.7 inches max LOP.