How do you know when to replace your car's spark plug??

im not really sure when the last time i replaced my spark plug was. but about how long do you go until you have to replace your cars spark plug? how much does it cost? i have a v6 3.4 cyl. also, i wanted to do it by myself, and its going to be my first time doing it, if i end up having to. any tips and advice guys??? thanks