A lot depends on your "style" or method of camping, regarding your propane consumption. For example, I just filled both of my tanks before my 7 days out a couple of weeks ago. I bought my camper in late March of 2010 and this was the first fill. So, in nearly 2 full seasons of camping, I had used one and 1/2 tanks of propane. I have 2 - 20lbs tanks.
In my case, 2 - 30 lbs tanks would be way overkill, as I could actually get by one one 20lbs (most of the time); just filling at the beginning of the season. Two 20lbs tanks are more than I need, but nice to have.
With that said, most of my camping is at State Parks; most of the time with electric available. I just don't use much propane. I don't cook inside, run the water heater & fridge on electric and, use a small electric heater.
I am fortunate to have a propane company a couple of mile up the road, so refilling is not a big issue. However, in a pinch, there are several places (around the Indy area) where one can simply swap an empty 20lbs tank for a full one (gas stations, super markets, home improvement stores, etc.). All things considered, two 20lbs tanks fit well in my situation.
During my week out (all with no hook ups), I did not empty one tank.