When can a trespasser occupy a sight and gain legal ownership of it?

Why are you asking? Would this be property be in Siberia... Iran... The Congo? Important info since laws vary greaty across the world... and this site is international in nature.

If you are talking about the US then you are referring to taking ownership under the law of adverse possession. The requirements for gaining title by adverse possession differ depending on the state. Let us know which state the property is in and we can tell you the specific requirements.

I see we have a VERY rude poster that thinks we can all read minds as to WHERE this mysterious property exists... the fact they VERY rudely gave an unhelpful for asking tells me they think they are wanting to know how to steal someone else's property without having to pay for it.

Its also clear they can't grasp the fact that whatever country they live in isn't the only english speaking one in the world... are grasp the fact that people of NON-english speaking countries frequent this site...

Personally I hope they find themselves sued or in jail for criminal tresspass by the rightful owners. People that ignorant deserve it.

I guess I didn't ask the question correctly. I read about squatters in the paper and wanted to know more about it. I feel so sorry for the people who are under water on their homes or are going through foreclosure. I was interested in knowing if a squatter cold take over their home.
I am not a squatter nor do I plan to be. I am a retired teacher from Florida and am blessed to own my home. :)
Thank you, Lisa from New Jersey, for your response.

It isn't so much that you ask the question incorrectly, but more that you asked for a simplisitic answer to a complex issue. No they can't just take over someone's property. Do some research on adverse possession. Generally adverse possession is used to take over a portion of a property that has been used by a neighbor. But it might be used to take possession of a whole property.

However, the rules for adverse possession make it difficult for someone to just trespass on a property and claim it.