What I would prefer is to work as long as I need to and when I come to a logical breaking point, I go home. Some days that may be 4 hours, other days that may be 12. Some things I can't easily break down into chunks that easily fit into 8 hours and not have me lose my place by going to sleep. Some days I get into a flow and I don't even realize I'm hungry until it's dark again and I suddenly realize everyone's gone to bed and left me on my own. Other days I struggle to figure out what I'm doing and the more I fight it, the harder the struggle becomes. It's not because of a lack of discipline or lack of want to work, it's just that I struggle to connect dots that when I'm on form just connect themselves.
On days when I'm on form, I'm unstoppable, a machine that just churns out working, tested, simple, elegant solutions to problems. When I'm not on form, I can't figure out how chew gum without gagging. This seems to hinge on how much sleep I've had, which directly impedes the amount of time I have in a day... finding the balance between enough sleep and enough time to get work done to a high degree of quality is a constant struggle
So I don't think changing the length of the work day for me even enters in to my psyche. It's always either far too long or nowhere near long enough... it would be the same if the work day was 2 hours long or 22 hours long.
That's what it's like for me, I can't speak for everyone else.