Thank you for contacting us with your question. Because of the various types of cooking equipment and the many choices of in-line and exterior vents available to ventilate a kitchen, there is no one maximum length to recommend. Elbows always add static pressure and reduce effectiveness of the vents. In order to get an answer to your question, you would be able to measure your intended duct run, count your 45 and 90 degree bends and call our Technical Service department at 1-800-637-1453. If we know the type of cooking equipment (BTU surface output) and your duct size and plan, we can advise you properly. We appreciate your questions.