We have the same issue, here in rural Maine, no NG. We put in two 200 gallon LP tanks (also use the LP for the gas fireplace), it was installed last March and the gauges on the tanks haven't moved off full. It gets run for about 10 minutes every week, to test the system and charge the start up battery.
As far as running time a single 100 gallon tank will give you about 43 hours of run time at 100% usage and two 200 gallon tanks will give you about 86 hours, but will last a lot longer as the demand drops.
Hmm, that's good to know. My house is not very large (3 bedroom garden home, less than 2k sq ft). My fireplace is a false electric one, so no gas logs, the generator and maybe my grill would be the only thing on it. Being in Alabama my main problem is outages due to tornadoes/hurricanes, but we are due for a 20 year snowstorm and the last one knocked out power for a week.