How long does a cat's labor last... she had had 4 and is still in labor and it has been 3 hrs since the last one how long before I start to worry?

Yes, if she's still having contractions and no kittens are showing up, it's time to get her to a vet. There should only be about an hour, at most, between kittens. Anything more than that (IF she is still contracting and hasn't stopped labor) could be a sign of a problem. Take her and the kittens to a vet, otherwise you risk losing mommy and any unborn babies.