My DF mentioned something similar and I asked who told her. Her BIL who has not even started practicing as a pediatrician. I couldn't help snorting at her, because I guarantee that he was under the assumption that sleeping through the night was 10-12 as opposed to the 6 hours that typical experts are referring too.
To answer your question, when mine were that age they were sleeping about 3 hours at a time.
Granted they were smaller, but I think the idea of them sleeping 10-12 hours at that age is really delusional.