Zippy made a good point about what is normal for a running (tactical) stock and target shooting stock is very different. I gave my LOP is an example for someone of my size and build, but should have said that is for targets.
Anyway, I reread your question which is if a 14" to 15" LOP is normal. Asuming you are of normal size and build (5'8" to just over 6'), I believe that is "normal" for most purposes. It is not "normal" for a combat gun as Zippy and Creeper point out.
I had thought of getting something like your Knox for my 870, which is the same model depicted in your picture, so that the women of the house can use it. If I did, I would get the recoil absorbing stock and keep it set on the shortest setting.