Low to mid 14s is common... however, many prefer a shorter LOP with heavy clothing, tactical web gear, vests... and those that prefer a more squared to the target stance rather than an angled one.
It's a bit of a subjective thing... there are no hard, fast rules.
I have two personal "rules":
1. Using a nearly square to target stance, which I prefer, can I bring the shotgun up from a low ready position quickly and on target without accommodation or adjustment?
2.Can I shoot several boxes of full power buckshot without discomfort, and at the end of the day, does rule #1 still work?
More often than not, I personally end up with a 12.5" to 13.5" LOP.
Shoulder Drive Nicholson Club