Till it runs out! Sorry, couldn't resist. Don't know for sure, been awhile since I did a gas burner. I would think several cooks anyway. I had a 100 pounder on my big 200 pound ss smoker, used to get about 10 plus cooks out of that in the winter, more in the summer, course, that's a mighty big smoker.
You might weigh your cylinder, do your average cook, record how long that was then weigh your cylinder again. That would give you a rough idea of how long it will last. Good luck!