From Yahoo News: Ex-CEO backs limit on leaving passengers on tarmac

 A former CEO of American Airlines on Tuesday backed imposition of a three-hour time limit on how long airlines can strand passengers on airport tarmacs, but he also warned of unintended consequences.

Even three hours waiting on a tarmac seems too long.  But, at least this would prevent those horror stories of people being kept on planes with no food and overflowing toilets.