[jdlegg]

I would say at most 8. I don't necessarily believe in working less than 8 hours per day has a particular benefit, but I don't really have evidence, even anecdotally, for that belief. 10 or 12 hour days are OK as one-offs, once-in-a-blue-moon (I mean that literally). But doing that for many days in a row has major negative impacts on my productivity and my general mood.