how much should a 17 year old run a day?

It really depends what your training for, or if you want to do it for leisure.

I'm a runner and I'm 17 and I do different types of training (eg long runs, short runs, rep sessions)

I train 6 times a week (I started at 3 and slowly worked up to 6) and here is a typical week.

Sunday: 60-80mins

Monday: 30-45mins (or maybe a swim)

Tuesday: rep session, (up to 75mins with breaks)

Wednesday: 35-45mins

Thursday: short reps (up to 75mins with breaks)

Friday: REST (or maybe a swim)

Saturday: race or train 45-75mins depending on what session.

But to be able to run a lot without getting injured it's good to do things like core strength or swims. By swimming it gives you endurance without the hard impact on your legs, it also works other muscles you don't particularly use when you run. Strengthening your core means you will be able to run more for longer without getting injured or with a less chance of getting injured,

Are a runner or just want exercise?

Runner = Probably work yourself up to an hour a day

Exercise = 30 minutes but build up to more if you wanna build muscle

Depends on how much gas the Free Candy Van that is following you has.

As long as you can, that's when you know you ran enough