I'm told it can vary. And if the cat is impregnated more than once, there can be two litters. The first five came out very quickly, six and seven took their time - still waiting for eight (and maybe nine).
Fraternal twins (dizygotic) develop separately, because they're two different eggs. But I don't know if it's possible to date foetuses to two days apart. Or why anyone would even try! But I only know germs, not babies.
What's up with this glass? Excuse me? Excuse me? This is my glass? I don't think so. My glass was full! And it was a bigger glass!