Hey...this site has some good info for you:
http://www.japan-guide.com/e/e6400.html
Honestly, 8 hours is a reasonable amount of time to leave the airport, but I wouldn't recommend traveling all the way into Tokyo, which would be a minimum 2 hr round trip on the train! You could spend a nice full day in Narita, check things out at a leisurely pace. Best of luck!