Nothing to do with protocol. If your group doesn't fill up in the first seconds, waiting is pointless: no one will join.
I am pretty sure that groups are queued to be filled. Once a group is made the game goes to the next group and so on. Which could explain why some groups can get 4 people in a couple of seconds and why you can get stuck in a group of 2 forever.
So I wait about 20s, if no one else join you have better chance looking for another group.