I feel my best on 5-7 hours sleep but every 4 or so days I need to sleep for 10-12 hours to keep it up.

same for me.. if i get more than 7 or so i feel dead all day unless i can get like 12+ which happens almost never.. i find my sweet spot is 6-7

At least 7 hours is something that everyone should shoot for. Go on google scholar and search sleep deprivation and blood glucose. Not sleeping enough causes insulin resistance and elevated blood glucose levels.

That's probably a good number.  I think there are people out there who legitimately don't need that much, but I think most people do need 7-8 hours.  I find once people start getting 8 hours consistently (meaning for several weeks) they find that they no longer function well on 6 hours.