Let's do the math, shall we.
8' x 8' = 64 square feet (exterior dimensions)
Height is roughly 12' with the main structure having an area of 512 cubic feet and an attic which has an area of just under 130 cubic feet for a combined total volume of roughly 650 square feet assuming the exterior dimensions, so it is actually less.
This an excerpt from the blurb for the unit I have.
Fits 6" Round Duct 257 cfm Galvanized steel housing features baked .......
This would indicate that the fan empties the entire units worth of air every 3 minutes or less.
So far, no complaints.
Above 90F outside, it doesn't matter what you do, the interior climbs above 95 and the plants don't like it.