Indoors, there are a couple contenders for me. Running around my high school gym was probably around 15 laps per mile, and I certainly ran a single mile as a warmup pretty regularly whenever we had indoor practice due to ice or too much snow. Probably ran a couple miles at once sometime, but not sure if I ever did more than that.
I have done stairs in a tall building on several occasions, which is practically like a loop depending on the setup. One time was in Colombia just before heading to the airport for my flight home - probably only 15 minutes (taking the elevator down, so maybe 10 minutes actually running up the stairs). Got my heart rate really high and felt like it was a good workout... but strained my achilles shortly after getting back and I think the stairs were a factor.
I have regularly run outdoor "infinity loops" - diagonals on a grass field - often on a loop that is around 200 meters. Most of these runs were 20-45 minutes, but have gone over an hour several times. Typically really boring, but my legs recover quite well from those runs!