I rode both fixed an single gear freewheel for years, including multiple century+ rides in rolling terrain.
But if you mean a single gear on a derailleur bike, easily 50+ miles, either when a cable broke, or on the Montauk Century, where there's no need to shift except for changes in headwind.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.