Well some people consider that middle stage begins/early stage coming to an end when half of the original number of players remains.
You can recognize the middle stage begining, also when you notice that average stack is around 30bbs.
Middle stage lasts to bubble situation
September 2nd, 2017, 3:07 PM
Poker at: ACR
The way I usually think about is that the early stage (especially in freerolls) last until the first break, which is when most of the donks and all-in every hand people are gone and can't rebuy any more to cause you headaches. The middle stage goes from the first break to the bubble which I consider 10-20 people away the money depending on how big the MTT was and how many spots get payed.