Here is a table showing air changes recommended for various buildings.
As an example, if your building is 30,000 square feet with 33 foot high roof (no attic) then the total volume of air is 1,000,000 cubic feet. Based on the table below nearly every space has a recommended change of at least 4 times an hour. Rooftop ventilators come in sizes that are rated in CFM or cubic feet per minute. 1 million cubic feet in 15 minutes is 66,666 CFM.