Without looking anything up, I remember when I was a kid that the Vanguard satellite, which has an orbit that is quite high (I think the lowest point in the orbit is about 600 miles) was supposed to stay up for over 200 years.

I don't know how much of that longevity is due to the altitude, and how much is due to the almost non-existent drag of the little sphere.

Still, I imagine that geosynchronous satellites will stay in space a VERY long time. They will drift out of their proper positions, and will suffer degradation from micrometeorites and radiation, but otherwise they aren't coming back to Earth any time soon. I would guess a lifetime in the thousand-year range at least.