Just curious if there's a set of guidelines to determine when a player can call the opening pretty much done, and move into the middle game. I know some rules of the opening that would apply are
All pieces developed
And by those definitions, it's easy to see that white may begin a middle game strategy while black is still in the opening phase, or vice versa.
I'm pretty sure there is no hard and fast rule such as
By move 10 all players should be thinking middle game, regardless of position
As all games are different (sometimes one side plays aggressive and there may be no time to castle).
I see a related question on the line between middle game and endgame, but not this one.