How long does a baseball rain delay last, before the game must be called off?

A rain delay must last at least 30 minutes, before the umpire cancels the game. According to Major League Baseball, until the beginning of a game, the home manager reserves the right to delay or cancel the game. After the beginning of the game, this responsibility shifts to the umpires.

When the home team is leading, a game becomes a regulation game, after the first half of the fourth inning. In the event of a tie or visiting team lead, the game must last until the beginning of the fifth inning to be considered a regulation game. When a game is cancelled, the game is continued at the exact point as when the cancellation occurred.