I usually do my pokes in batches of five. One poke is the fainter and also wears the experience share to keep the experience of the others down if they're too close to level 50. Then I choose the pokes to EV based on the stat that needs doing. They all switch into the fight, one after the other. That way they all get the same EVs from fainting the same pokemon.
But since you're EVing for other people, you may have pokes that don't need the same EVs, so that would make it longer to do.
I got to quit chatting. I'm making too many mistakes on my RNG because I'm not paying attention to the timer. I'll talk to you later.