D.,
For me, this is an intensely personal question. The answer depends, I think, on your answers to two sets of questions. First is normal "mom" stuff, like "how much time does he need to be home for homework, sleep and family togetherness stuff"?
The second set of questions is more difficult:
1. Do you know and/or trust his friends? If not, then tell him that part of his being allowed freedom is knowing you can trust both him and the people he's out with, and tell him to bring his friends around first - maybe for dinner or just to hang out sometimes - until you get to know them.
2. Do you know where they will be hanging out? If not, you need to tell him that you always need to feel safe about where he is, and ask him to clarify this for you. If you have any doubt about the location and/or people in his hang-out, ask him if he's willing to take you there and show you, at least once! If he says "no," beware.
3. Does he always tell you where he's going, who he'll be with, and what time he'll be home? Communication is very necessary from this point onward. If you think it's bad that he wants to be gone now, just wait until he's much harder to track down because he's got wheels! Now is the time to train him to ALWAYS tell you his whereabouts.
4. Will there be grown-ups somewhere near? If they are going to a safe neighborhood park to hang, in the middle of a Saturday, that may be ok. If they are going to a ballgame or movie and will pretty much be sitting, that may be ok. But there are places you won't feel comfortable unless they're chaperoned. And there are kids (either your own or one of the friends) who are always looking for trouble, and should be chaperoned no matter what. It's your responsibility as a parent to figure out which kind of kid yours is, and your kid's friends are.
I have a great story about this! My older daughters are now 21 and 23. My younger one was never where she said she was going to be and I was constantly getting into my car to hunt her down. The older one was much more trainable - or maybe she trained me. Here is the story - you be the judge.
One Friday evening I had a dinner party with a group of friends. After the meal, the ladies were sitting in a group talking (the men were in a different group, of course), and my oldest daughter walked up to me. She was 17 at the time and had a car (her dad's old one). She said, "mom, I'm going over to so-and-so's house. We're going to hang there and talk until about ten, when so-and-so gets off work, and then we're going to head over to Coffee Plantation (a coffee shop in phoenix with a guitarist on weekends) and listen to music. I will be home around 11:30, if that's ok with you." Then she kissed me goodbye, started to walk out the door, turned around and said, "oh, yeah, I have my cell phone so you can reach me if you need to, and I will only have one other person in my car because so-and-so2 will be driving herself, so don't worry about that," and walked out the door. My friends (who also had teens) looked at me and said, "WOW. How did you train her to do that??" I laughed and said, "I think it's the other way around! She could be out prostituting for all I know - either she's perfect or she's learned how to pull the wool right over my eyes!"
By the way, my other daughter (now 21 and of course nearing perfection), was the exact opposite. Wherever she told me she was going, that was not where she went. Her friends were nice but troubled kids (as was she), and some of their parents were not even appropriate chaperones. This child got used to my pulling up to check on her whereabouts, insisting on meeting the parents of her friends, clearing out parties held in parents' absences (my daughter once marveled at how efficiently I'd cleared out a party full of kids - the girl hosting was in 8th grade and the place was full of high school boys taking advantage of an empty party spot!). She finally more or less gave in because she knew I was going to stay on top of her. Now that she's older, she's actually thanked me, but at the time, it wasn't fun for either of us. Once, when she was 17, she disappeared with her (fairly trustworthy) boyfriend for a short hike and didn't turn up for 13 hours, and didn't answer her cell phone, etc. I actually started calling hospitals. One of the ladies on the other end said, "ma'am, I'm sorry, but your daughter is not here." I laughed and said, "Don't be sorry. I don't WANT to find her in the hospital." I can laugh at all this NOW...