How long can newborn kittens survive without their mom?

I got home only to find a stray cat in my kennel on the back porch. She had given birth to 4 kittens. I went outside to feed them like I always do (them being random stray cats) and she bolted, leaving me bewildered until I heard the meowing. This occurred around 6:30pm today and the mother hasn't been back. Using gloves I changed the towel because it was bloody and put a heating pad in under it. I rubbed each with the bloody towel first and left it out there. The mother hasn't been back and I'm beginning to worry. It's getting cold and it's already under 50 and it's supposed to get to 36. What do I do?