breaking hands

[Thom Coste]

No time limit on separating the hands after stepping off.

That having been said, however, he can't just step off and stand there. The pitcher has only 20 seconds after receiving the ball to do something - pitch (which, as you said, will require him to separate his hands before stepping back on the rubber), make a play or attempt a play including a legal feint - any of which will naturally cause the hands to separate.

Most umpires don't put a stopwatch on this 20 second requirement. Most pitchers don't abuse it. But I'm sure that a pitcher who persists in delaying the game by constantly stepping on and off the rubber - with or without separating his hands - will be corrected, up to and including being penalized appropriately (ball on the batter).