When did you toddler start staying dry during sleep?

Just curiosity really. We have been talking with some friends of mine and they were all shocked when I said that my girl ALWAYS wakes up with an empty diaper since she turned 12-12.5 months. She is 2 now. She has been having dry diapers every morning, then she would have a huge pee 10-15 min after. Now we do that pee in the potty. She also used to nurse multiple times at night till 15 months, then 1-2 times till 18 months. Still no pee over night. I always thought this is the norm, until this discussion.