What is the amount of time that someone can live in your home in NJ before they can claim squatters rights?

There is no time limit for this - as long as it's your home, it's your home. Now, if you have clearly abandoned the property and the squatter is acting as if he owned it (he pays utilities, property taxes, etc. you don't visit the property, and you don't protest his assertion that the property is his) then it can get tricky, but I believe that this activity would have to go on for a substantial period - 20 years or so - before you run the risk of actually losing possession. To avoid this, all you need do is notify the person in writing that the property is yours, and/or perform actions that establish that you have not abandoned it (mow the lawn, walk the property, pay the taxes, etc).

I looked into this years ago when my neighbor started to regularly mow a section of my property that abuts his back yard. This is in an area that I virtually never use or even walk on - so I have no objection to him doing this. But my attorney suggested I make a point of walking on the area at least once/year, and I also sent the neighbor a copy of a survey that shows the true boundary.

Squatter's rights no longer exist. It has been replaced by the law of adverse possession.

In NJ the time for ownership by adverse possession is 60 years for uncultivated land and 30 years for other real estate.

In addition, the person must possess the property without the owner's permission for that entire time. If they have the owner's permission to live there then there is no claim for adverse possession. All you have to do is give them something in writing giving them permission to live there. If you don't want them to live there then start following the procedures for eviction.