On a 1999 454 mpi with 200 hrs. What should the compression be?
How many HRS. should it be good for, on average? More than a 1000 or less?

I think your compression should be around 150 psi. My 454 Mag puked after about 550 hours. Keep an eye on your riser/manifold gaskets. That's what killed mine. I ran in salt a lot and ran hard.

150-160 psi

Consistent numbers among cylinders is important too.
Don't be alarmed to find 140s.

Should the motor be at temp. when checking?

Yes, and throttle at wide open.

So the motor at temp then W-O-T then check?
I`ve never heard about the throttle. what does it do?

If the throttle's not open, air can't get in. If air can't get in, it can't get into the cyliders to compress.