I'm going to posit that the statistics that are being quoted (adults need 7-8 hours of sleep per night) are probably based on healthy, non autistic adults.
And I think that makes a huge difference.
Now, I can't say much about everyone whose got ASD and how much they should be sleeping. That study (so far as I know) hasn't been done yet. What I can say say is that unless the lack or excess of sleep has become a problem in someone's life, or they have other symptoms indicating an energy or heath challenge, then they should sleep as many hours as they need to feel rested.
I've had atypical hypersomnia for most of my life. For months I will fall asleep very quickly, then suddenly be up thinking half the night, totally unable to get to sleep. I tend to sleep for 9.5 hours, but when I've been very depressed, stressed out, or experiencing excessive social demands, I will totally sleep for 12-14 hours a day.
I won't say that it is normal to sleep 12 hours a day, but I think that it is important to remember that Aspies are not "normal" in many ways, and so long as they are getting rest, and are happy with their sleep schedule, then I reckon that what is "normal" isn't all that important.