I want to get a lot of emeralds from a fletcher and always trade 32 or 31 sticks for 1 Emerald. This is pretty easy and always works 8 times, because then there's a so-called "offer lock" that prevents you from exchanging more sticks. How long do I have to wait until it is over and what is the fastest way to get to Emeralds?
Until he uses his professional block. Then all trades will be unlocked again.
He does this every few minutes, provided it is working time (i.e. During the day) and he is standing next to the block.
The fastest way to get to Emeralds by trading is to zombify and heal a farmer until he has all trades on 1. And then deal with him pumpkins, carrots, potatoes… 1: 1.
If you need a lot of Emeralds I recommend a raid farm. They are even more efficient than acting.