Well, looking back through the responses, I see 3-4 mo on the low end to 1 year on the high end. I imagine it will be something in the middle. "At least a year" sounds like a stretch, based both on my experience and the responses in this thread, but it'll last awhile.
Your high pressure gauge won't move from around 800 psi for the first 90 or 95% of the life of the cylinder. After that, it will begin steadily dropping. At that point, you'll have a few days to a couple weeks till you're completely out of CO2.