Will replacing the glow plugs on my 2000 F-250 7.3 deisel help with starting on cold mornings ?

Yes, that's a common solution to hard starting. It also helps to use a HDEO full synthetic diesel engine oil.

How many miles can you get before having to change glow plugs (mine has 100'000)

It varies upon climate--they last longer in warmer states, like Texas. Here's how to change the glow plugs:

With the engine "stone cold," disconnect batteries, remove valve cover, disconnect glow plug connector, remove glow plug with a 10 mm deep socket, install new glow plug (torque to 13 ft.-lbs.), and reattach connector. After finishing the remaining 3 glow plugs on that bank, clean valve cover, install new gasket, and reinstall valve cover (torque to 97 in.-lbs.). Do the same thing for the other bank of glow plugs and reconnect batteries. Not a difficult job--should take several hours.

Obviously, glow plugs warm the fuel, block heaters warm the engine, and synthetic engine oil helps the engine turn over easier in cold weather, saving the battery in the process.