Bought a Nikon D200 with a 66k shutter count? - 1

For $250 on Amazon, how long should I be fine without having to worry? It was from a studio photographer so I assume it was taken well care of.

I shoot about 5-10k a year

Chances are it'll work for many years:
http://www.olegkikin.com/shutterlife/nikon_d200.htm

It's impossible to give an exact answer, as with anything sort of statistics you get a large variance. It could be the camera that dies at 70k. It could be the one that lasts over 1 million. There's no way of knowing until you get there.

The shutter counts are only a good tool for getting a better price for the camera

Different cameras are stress tested to different numbers of shutter cycles. You will NEVER put your shutter through that kind of stress unless you end up machine gunning your subjects. That will put a lot of stress on your shutter.

I had an old Nikon D100 that had over 200, 000 shutter cycles on it when I eventually sold it. The new owner has added many more over the past two years and it is performing just fine.

Enjoy your new camera and do not worry about the number of shutter cycles on it