It has a lot to do with mood and willingness... If you resist tickling and are not in the mood and are below average ticklish, I doubt you are going to get much in results.
I'm not very ticklish myself, but if I get in the right mood, the right setting, and I tell myself that I am ticklish and can be tickled, I can be tickled in an way that will bring good twitching and light laughter. Pretty good for an under average ticklish person.
I also think you shouldn't expect to be reduced to a tickled mass of laughter unless you have tickle spots you are not aware of or unless you are tickled by an exceptional tickler who can get you in the mood and work your ticklishness to the max.
My wife is not so ticklish, maybe average or just under average. She can pretty much shut it off if she really wants to. But in the right mood, the right setting, I can get her to an above average level of ticklishness with greater laughter and excellent twitching.
As for the question of tickling long enough to break a person, I think if you are talking about a person who "keeps it in", then yes, you could break him or her (our be broken yourself) over time. But if you are talking about people who just shut it off and make you feel stupid as phat dog mentioned, I think it is pointless unless you find a tickle button that person is unaware of or is defending.
That's my point of view.
