It varies
This depends on a lot of things. For one, it can depend simply on the ticklee, and how ticklish they are; some people are just more ticklish, or can feel ticklish, longer than others.
The tickler can also be accounted for. If you tickle someone lighter first, it can build up to more intense, longer lasting, ticklishness for example. Switching locations instead of focusing on one spot is another key technique, like many others have already mentioned in this thread. It's also about the way the tickler tickles.
In my own experience, I've noticed that if I focused in one spot too long, my ticklee would laugh/giggle less and less, and become less ticklish there. I've always gotten a much different reaction when I changed locations though.
And as I've said before in a different post, tickling is all mental. You can often get a reaction from someone when you simply tickle the air over their body but not actually touch them. It's the thought of being tickled that gets you, even if there's no physical contact. Just like how you usually can't tickle yourself. Yet you have someone else tickle you and it's a whole different story! So a ticklee's mood can also be a factor in ticklishness, as that is a state of mind. Ok then, I'm done analyzing, lol!