How often should a restaurant review its P&L?

Weekly, in a short disciplined pass — with the month-end statement kept for reconciliation, not discovery. A cost line that starts drifting in week one of a month has run for four to six weeks before a monthly review can even see it, and the cause has usually gone cold. Twenty minutes a week reads five lines — sales, food cost, labour, occupancy and the platform's cut — against the published ceilings while there is still time in the period to act, and ends with one action, not a list.

From Five Nights to a Tighter Restaurant P&L: Food Cost, Labour, Rent, the Platform's Cut and the Weekly Read — the full reading, with the method behind this answer.