Most markers will give around 800 shots in the summer, and around 500 shots in the winter. Temperature is important to co2.
When i was using a mag on co2 i would change bottles at 500 shots in the summer to avoid drop off.
On a personal note, i never push the fills much above 3lb 2oz, as with the mag i was getting problems with liquid being drawn.
If your marker needs liquid then, yes 1000 shots would be about right.
The shocker being a heavy user of co2, does suffer from liquid being drawn, so i would be sensible when filling, and expect 600 - 800 per fill.
Be carefull about freeze down with the shocker, when shooting fast.