How often should I raise prices, and by how much?

Re-cost the plate whenever a supplier moves a price — within the week, not at the next menu print — and review the whole menu on a quarterly rhythm with real sales data behind it. Raise in small, dish-specific steps rather than across the board: the menu-engineering method treats a reprice of up to 6% of price as defensible on a popular, thin item, and protects the stars rather than fiddling with them. A menu that has not been re-costed in a year has repriced itself downward without anyone deciding.

From Cost-Optimised Pricing Makes Restaurants. Reckless Pricing Quietly Unmakes Them. — the full reading, with the method behind this answer.