It's the age old question, "How long is a piece of string." It all depends on conditions for your propane use. If your sucking hard with your furnace and generator, not so long. If you have solar, the weather is warm and friendly, and your not baking bread and cookies every day, your run time with a 5 gal aux. tank is appreciably lengthened. Though I've never actually tested the length of use for my 5 gal aux, I would guess it to be a week or better.
I suggest you do a few "millage" tests on your current propane fixed tank and that will give you an idea of how much your consumption is in given circumstances.
"If everything seems to be going well, look around, you've obviously overlooked something."