I think of something as "flat" if it's the kind of course I'd seek out to do a fast,
steady tempo speed. The flattest out-and-back course for more than 10 miles (it's 12.2 miles) that I can make starting in Ithaca, NY is +/- 27.1 feet per mile (it has a few rollers with above 6% maximum grades) and the flattest loop that I've created that's longer than 10 miles (it's 16.53 miles) is +/- 25.2 feet per mile, but it's got a couple of short back-to-back rollers with 10.2% and 8.4% maximum grades and almost all of the elevation change occurs within two stretches of 5 miles and 3 miles (plus the road surface is sub-par to ridiculously awful for about 4 miles). So nothing in town is what I'd call conducive to fast, steady riding for the whole thing. You can make short loops that are flatter (mostly downtown) but they have way too many sharp turns and potential stops to be good for a decent-speed tempo ride.
I have to go about 17 miles from my place to get to it, but riding on this road boosts the speed by a little over a mile per hour vs. the courses that start and finish in Ithaca:
www.strava.com/activities/1835528102
Strava says the elevation gain is 15.8 feet per mile and the road is only mildly bad in a few half-mile-long spots. I went out and back twice on that recorded bit and stopped short of where I started on the second return (needed to get to a C-store for a water refill or I would have had to go 21 more miles to the next store), so it was basically 23.7 miles for the first out-and-back on that ride. You can go a few miles farther SSE toward Owego than I did and still stay just as flat, so that means you can get 30+ miles on one out-and-back. You can't get 40 miles without at least two U-turns or without encountering a grade above 6%, but it's the best you can get around here if you want to stay on flat terrain or very shallow slopes. The regular state TT course is a little farther South of this road and is just about as flat, with an elevation gain of 16.8 feet per mile. It's pretty fast.