How long is a cat in labor from the first contractions until she starts giving birth?

Normally half an hour. But she can have early contractions that stop if there are a lot of kittens and she's having trouble carrying the pregnancy, or if she has poor diet, (pregnant cats eat at least 3 times what nonpregnant cats do), or some illness or other medical condition. But if her water breaks, they're definitely coming in half an hour.