Settle plates are poorly quantitative - you can try different times to see which give you the most info in a qualitative sense.

Thank you, qualitative is good but I am asking in a quantitative sense

If you've a controlled environment and protocol with specific biological target, decent reproducible, quantitative data can be obtained with air samplers. There are a number you can find on the net (e.g. https://www.bertin-instruments.com/product/air-samplers/coriolis-micro-air-sampler/). Suggest you contact suppliers and decide which is best in your app.

Thanks Phil, your answer has really helped me.