I don't really know what my ideal sleep length is, being a chronic insomniac since basically birth. I never really learnt how to sleep through the night, although this year, despite all the craziness and OTT emotional drama, I have had a few nights here and there of more than 6 hours of sleep. And I even had one or two nights of 10 hours of sleep just before some extreme major drama (eventually made sense of that as being some heavy emotional processing).
I can function long term on 4 hours of sleep a night (although I do feel it after a while - it's just a bit harder to think clearly), as long it's quality sleep, ie not waking up at all during that time. 3 hours of unbroken sleep a night gets to me after a few weeks and I start to feel sleep deprived. 3 hours of broken sleep or less than 3 hours and I really start to struggle after a couple of weeks and I'm not really functional (ie my ability to think at work starts to really suffer, and I start to get physical fatigue, my hands start to shake during the day and my stamina starts to drop).
I suspect I would do best on 6 to 8 hours of unbroken sleep a night, but I don't know for sure cos I've never really experienced that.
There is a plus side to all of this - more alone time for processing. Sometimes I think that's what's going on when I go through long phases of not much sleep.