How often should I count stock?

For anything you are actively trying to control, weekly — not monthly. A monthly count tells you a problem existed about four weeks after it started, long after the cause has gone cold. A weekly count, on the same day each week, lets you catch a drifting line while there is still time in the period to correct it. High-value or high-theft items may deserve an even tighter watch.

From Restaurant Food Cost Control: Theoretical vs Actual, and How to Close the Gap — the full reading, with the method behind this answer.