How many hours in your studio working day?

I usually do edits in the morning from about 9 or 10 till noon when the band shows up. We usually track from noon to 7 or 8pm. I try to take a break in there somewhere but a lot of the time I totally forget because I'm in the zone. Rinse and repeat 6 days of the week. Gotta have at least one day off per week for sure.
 
When tracking/producing with the band present: Over 10 hours and my judgement starts to distort. Over 12 and things become counter-productive. 8-10h is normal for me.
When mixing/mastering by myself: I can gladly work 16 hours if I'm in the zone. All the spare energy saved from not having to kibitz, look after people etc. makes me go a lot longer without losing focus. So anywhere from 8 to 18 hours is normal.

In both cases, at least one day a week completely off the project is mandatory.
 
I'll do 12 to 15 hours regularly, you just gotta take some nice breaks every few hours. you gotta take a good solid food or drink or nap time break every time you start to say "good enough" or whatever. you can really go for hours and hours this way, the only catch is you feel like shit the next day.
 
8 hours is a tracking day for me. Any more and people generally just burn out, you start lagging more and more in subsequent sessions and eventually just feel like crap by the end.

Mixing is around about the same when I can. On tight deadlines for the last 3 albums though I've done a couple of 14 to 16 hour mixing days just to get it out in time.
 
I've noticed one important thing about long mixing sessions (maybe I'll be the Cap here, but maybe not).

If I have to mix lots of stuff and the deadline is near, I'm listening the mix for some 20-30 seconds, then I write down what I hear is wrong. Then I fix it in silence. Then I listen again. Thus, actual listening (and getting used to a possibly faulty mix) is shortened greatly then listening and tweaking at the same time. So, for me it is (30 sec listening, 3-5 min fix)xN , then short rest, switch to another project, repeat.

Also, I'm switching between the projects after several improvements in each.

And the references are helpful when I start losing attention. I usually take the "unfixed previous mix" as one reference, "raw mix" as another, and a couple of my favorite records in addition. It really helps to "reboot" the feeling of balance.
 
12 hours MAX.

Occasionally I go over that, but TBH even 12 hours is kind of a waste. You get about 8-10 hours of useful studio time in a day, after that you're just spinning your wheels or worse, you're actually making bad decisions.

People don't understand how fatiguing making music really is.

Christ, +1. Hovering around 8 hours is what I consider my personal "MAX OF EXCELLENCE".... hahaha, after that is diminished gains....
 
I usually do around 6 to 8 hours, but lately I´ve been doing up to 16 hours daily so I can pay my late bills. I am currently working with 8 (!!!!!) bands at the same time, so the only way to be able to do all jobs is overdoing it - not right, I know. But, usually after 6 hours, my ears are shot and my brain is done. After around 6 hours, I just work on my auto-mode and most things sound like crap. The auto-mode is for the clients that think I am just a record-pressing monkey :) Btw, this happens mostly on the studio I work on. My boss isnt a musician or anything near that, so he doesnt know shit about what works or not inside the studio. Most pro musicians around here also just want to receive their money , so they want to finish things quickly.