GM power is a wildly variable thing that there's no right answer for. The situation you're describing is pretty shitty and it makes it sound like your GM is, but in the grand scheme of things, GM fiat is a wildly fluctuating thing that varies from game to game, as trust grows.
The latter half of your question, though? When is it time to find a new game? That one's easy. "When you're not having fun." It doesn't matter why. It doesn't matter if it's the GM, another player, or even the game and its rules. It matters that, when you're playing a game, you're enjoying yourself. Games are supposed to be fun. That's what they're FOR. You play games to sling dice and tell cool stories and have a good time.
If you're not having fun, why are you there?