I believe a professional spray booth clears the air at a rate of around 3 to 4 times per minute if I am reading the info I have seen right. An article in the December 2006 issue of Auto Restorer recommended that for a hobbyist/homeowner once a minute was sufficient. What you'll need to do is calculate cfm you'll need in relation to the size of your paintbooth.
The formula appears to be:
W x L x H x 0.8 = CFM
So for example for a 15' x 20' x 8' booth you would need a fan capable of 1920 CFM. An attic fan should be capable of this as should a decent sized box fan.
