How long of a notice should I give my employer?

I am planning on moving back to my home state in about 6 months. I am basically at the bottom of the ladder in my current position, but I've seen that my employer has a very hard time filling positions like mine (2 positions open for almost a year now). While I like the people I work with outside of work, the reason I started seeking another position is because of how they treat me at work. I am afraid that if I give too much notice they will treat me even worse, but at the same time I would feel bad for leaving them understaffed for potentially a long time. (2 other employees that I know of are leaving for the same reasons, and I will be the last to leave unfortunately)