I rode from Seattle to Phoenix and back two up and pulled a trailer. For a little bit of gas insurance, I put a quart fuel container in my trailer. I never had to use it. After a 100 miles, I am always looking for a reason to get off of the bike for a short break. We averaged 500 miles a day and had time to enjoy the ride and didn't feel beat up at the end of the day.
Last year, I put the same quart bottle in my saddlebag on a solo 1300 mile weekend ride through Washington, Oregon and Idaho. I didn't use it then either.
I felt a quart of gas would get me 10 to 12 miles.
I won't discourage such use of these extra tanks, but in all of the forums that I have belonged to, many of the riders who setup for this end up removing the extra tank after their enthusiasm wains for putting so many miles between fill ups.
Plumbing it in so that it could be quickly removed so that it wasn't on the bike full time should be a MAJOR consideration.