In a standard three-hour Major League Baseball game, for approximately how many minutes is the ball actually 'in play'?

18 minutes

A 2013 timing study of three MLB games found 17 minutes and 58 seconds of broadly defined “action” during a three-hour game, but only 5 minutes and 47 seconds involved balls in play or runners advancing. The rest of the measured action included pitches, foul balls, and pickoff attempts, while much of the game’s inactive time occurred between pitches, batters, and innings.