It's more than just "hanging out for too long". You have to be clearly occupying the space, and you have to be improving it.

In California, "Adverse possession" requires a lot: You have to occupy a place continuously for 5 years, like how an owner would. You have to pay all taxes on the land for that time. You have to be clearly occupying the land, so it's obvious to all (including the owner). You have to be acting like you are the owner of the land: Getting & paying for electric, water, internet, trash pickup, etc..

So, there's a high bar that has to be met.