When I was 16 I walked like 10 miles to hang out with this girl.
tasted my first vagina so all was worth it
I'd say it is all dependent on gas and transportation. If neither are a problem then a couple hours a way is no biggie. I had a buddy who was a natural and he would just take buses to see girls, kid was a beast, I was so jealous of him and that jealousy is what ruined our friendship but I had to realize something, he was going after what he wanted where as I was just expecting it to come to me.
Also, eventually you will run out of room and options in your current stomping ground so you will have to expand your horizons, so traveling to the next town or next county over is no big deal ;]
personally, I'd say anywhere within 6 hours is fair game if you can afford the gas and you know it is a sure thing.
A crazy idea...if you know you have multiple girls in one state/area, plan a trip and see them all! (individually of course.) that would be fun!