A DoD study on the risk of catching the coronavirus on a packed commercial flight concluded that a person would have to be sitting next to an infectious passenger for at least 54 hours to receive a dangerous dose of the virus through the air.

https://www.washingtonpost.com/local/trafficandcommuting/defense-department-study-finds-low-risk-of-coronavirus-infection-through-air-on-a-packed-airline-flight/2020/10/15/e84aa092-0e30-11eb-8a35-237ef1eb2ef7_story.html