Crossing the atlantic in a B707 back in the 80s I experienced several hours of (what was for me) moderate turbulence, punctuated now and again by short bursts of pretty severe stuff. The constant banging into the arm rests bruised the top of my legs! I was sitting in the very last row. Later, when things had calmed down a tad, I visited the f/d and out of curiosity asked the Captain how he classified the turbulence. To my astonishment he replied, "light". I offered to swap places with him!
Don't sit near the back folks. You get the worst of it there.