At what point does the opening end, and middle game begin? 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
King castled
Rooks connected

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.
The opening is for "deploying." The middle game is for "fighting." So the middle game begins when the fighting begins in earnest.

Think of two armies marching to a battlefield. The opening is when they "leave camp." The middle game is when they arrive on the battlefield and get down to fighting.

If there is serious "fighting" a middle game can begin as early as the seventh or eighth move. On the other hand, the middle game can begin as late as the fifteenth move or so if both sides opt for a "slow" development.