These things are all relative. I looked it up on the auk website
It consists of the BR(M) series: 200 km, 300 km, 400 km, 600 km (
https://www.audax.uk/results/achievement-awards/randonneur-awards/ )
So from my point of view they range from "long", through "really long", to "you're kidding"
The next question is what does BR(M) mean. I think I know what it stands for - Brevet Randonneur (Mondiale) - but I have no idea what it means. If you check the auk website it is probably defined in terms of yet more TLAs. I think it's a "flavour" of audax that means the organisers have to do particular things to qualify.