I can only remember one flight with heavy and prolonged turbulence, it was a FRA-MIL on LH, on a 319. I didn't think it was particularly worrying but people were screaming and shouting for 'help'.
The captain made an announcement that we would soon be out of it, and asked those who were able to see the wings to look out. He then explained that wit the turbulence they were flexing approximately 3 metres upwards and 1 metre downwards, and that in very severe turbulence they could flex up to 10 upwards and 15 downwards, so that what we were in was relatively light and well within the tested stress limits of the aircraft.
I'm not sure if my memory is correct on those figures, someone on here will know, but even the panic stricken lady next to me found the announcement reassuring.
My biggest fright with turbulence was somewhere over Central Africa being awaken from my sleep by what seemed a massive bang, to find that we'd hit a patch of CAT and dropped a few hundred feet. That was scary, but over very quickly.