"Standard" varies by manufacturer, country, and stock style.
Generally, adult LOP is 13" to 14" in the United States.
Sometimes, 'womens' stocks might be 1/2" shorter.
Youth stocks are generally 11.75"-12.5".
I, personally, am just under 6' tall and have long arms. For the typical "American style" shotgun or rifle stock, my ideal length of pull is about 14.4".
That being said....
It's difficult to try to lump LOP into set categories, because it is influenced by many factors.
For example, my M38 Mosin currently has a 13.5" LOP. It feels just a little short, but is usable. On the other hand, my Rossi 62SAC has a 13.5" LOP, feels like a tiny little kids toy, and is sometimes difficult for me to get comfortable with.
The differences that cause that? (In this case.)
Grip angle / style.
And... believe it or not, position of the support hand.
For a lot of people, a straight stock (non-pistol grip) makes the LOP feel shorter. And the closer you get to a vertical grip, the longer it makes the LOP feel.
And, that ties in with the support hand problem. The farther out you grip a long gun with your support hand, the more the rifle tends to cross over your body. And, as such, the longer the LOP needs to be to compensate for your strong arm following the rifle.
Then you can take into account drop at comb, drop at heel, butt angle, sight heights, etc.... and get completely lost.
The best way to figure out what LOP works for a person, is to have them hold the long gun with their support hand where they want it, and then slowly build up the length of a short stock until the length feels right.
And if you're trying to figure out the LOP of stocks sold by a company... just call them.