...hm...It can take up to a day for another birth..but not in cats, I've only heard that with larger dogs..Normally if not in 4 hours you should go to the vet, but if one was a stillborn, and there's nothing, I'd be worried, this could be life/death for the momma kitten. But definitely, if not in 4 hours, definitely take her to the vet. I know nothing about cats in labor though.
Female cats and dogs should be spayed soon after the age of 8 weeks. Males should be neutered at 8 weeks of age, but both spaying and neutering can be done safely through most of adulthood. Some shelters are encouraging earlier spaying and neutering, which can be less stressful for animals. Younger animals also recuperate faster from surgery.