In my experience they don't wear off completely. After 5 or 6 hours in really bad waves I will start to get queasy. I should take another one but haven't yet.That's what I figured.. I'm more interested to see how I feel when they wear off. If I take @ ~ 5:30am, boat gets held up and leaves at 7(ish) and for whatever reason doesn't get back until 1 or so it will have worn off.
I might be ok... hopefully used to the sea by then. Like I said, last ride out I didn't take anything all day and I was pretty good. Not perfect, but not miserable either.