The shortest I could find was five years, most are ten or more. While I'm not much for squatters rights, it's stupid to leave a property abandoned for that long without doing something to maintain at least an appearance of a claim of ownership.
That said I don't think that's a problem so much. It's the court cost of evicting them once their in, even if it's during a two week vacation. It's not a cheap slam dunk process apparently even in that case.