Direct answer
Select airflow from enclosure free volume, expected temperature or altitude change, allowable pressure difference, change rate and the membrane's airflow at a stated test pressure.
Start with the pressure event
Define the fastest credible event: heating in sunlight, rapid cooling from rain, altitude change, battery heat or a production leak test. Estimate the gas-volume change and determine how quickly pressure must equalize without exceeding the seal or component limit.
Compare data on the same basis
Airflow is pressure-dependent, so compare candidate membranes at the same differential pressure and active area. Include adhesive openings, support layers and contamination margins. Validate the assembled vent because fixture and housing restrictions can reduce effective flow.