How Many Hours?

It depends entirely on the rider, some are comfortable enough to hit the track from the start, others should never get on a track. If you are afraid, then stay away, but a little nervousness is expected.

It also depends on the track. You shouldn't head out to a supercross style track a day after you learn how to ride.

As long as you can control throttle, brakes, steering, balance, and shifting with brainspace left to think about the changing terrain and obstacles in front of you, you should be okay.

When you do go to the track for the first time, if you haven't ridden for very long, don't expect to be railing berms and hucking huge doubles. take it easy!!! Hold your line and don't ride all over the track. Other riders will be passing you. if you want to know more about your first time at the track, just do a search. There are loooooots of threads with the same topic.

depends on rider, skill and nervousness.

Everyone can ride a bike with a bit of practice, but some will be better naturally. But the thing that's the same is if you have never ridden a bike before you will be at least slightly nervous.

I would argue that a tame track is better for a novice rider than an open trail. At a track you can see what you are going to ride. After a couple of laps you will know what areas to go slow on and where you can push it.

On an open trail a beginner can get in WAY over their head in an instant when that nice, wide, smooth trail turns into a wash out. Tracks typically don't have big rocks that you don't see until you are right on them. Everybody goes the same direction on a track and you don't usually share the track with quads or off road trucks.

If you crash or break down at the track you won't be 10 miles from your truck.

Rod