I have been asked that question many times on this website, and since there appears to be a good deal of interest, I have added a page to this site dealing with how long it takes to fill an air tank.
Then, you apply the information to the following formula:
That gives us:
100 x 80 = 8,000
440 (7.48 x 14.7 x 4 rounded up)
= 18.2 minutes
Folks, you are welcome to use these calculations yet be a bit cautious. While the information is sourced from a number of reputable locations I cannot be 100% sure of the accuracy, and therefore, you use this information at your own risk.
If you need to be 100% sure, get an engineer to help you.