The chance of a 6x31 spawning is 1:887'503'681.
So assuming there are:
-3 servers
-1000 people catching pokemon all the time on each server (catching all seen pokemon)
- a pokemon spawns once every 5 seconds
That would mean 600 pokemon per second caught. A day has about 80000 seconds, so we get a total of 48'000'000 pokemon per day.
So if there really were 1000 players per server encountering a pokemon every 5 seconds 24/7, they would see a 6x31 once every three weeks on average.
But considering there are much fewer encounters per day, and most of them are just fought out/run away from, I think it is quite unlikely that there exists a full 31 poke on PRO ;)