Gene,
What we have found is that our 4, 6 pound tanks lasted us around 5 months of full time cooking for both of us. That is in the Sea of Cortez, HOT weather most of the time.
We carry 3 of the tanks in the spray hood storage, now called a “Greg Box” by the factory . 2 on the port side and 1 on the stbd. The stbd. Side bottle is a tight fit. We have one bottle on the stern rail in a SS fender holder. It fits great and makes it easy to change.
We also found that the 6 pound bottles are very easy to carry around! Much better than the 20 pounders.
We recently decided we did NOT need to carry that much so we have cut back to 3 bottles, 2 in storage and one in use. We now keep our Baja Filter in the Stbd side where we used to store the 4 th tank.
Greg
Photo shows the tank "in use" during a passage....