everyone gets 1 vote
Two hour sittings seem reasonable, even if you're doing 8 hours a day (which seems like an insane amount of time o_O, to me at least).
I would have to wonder though at what point you would no longer be gaining any benifit, After four hours I don't know if I would still actually be learning anything or improving as opposed to just going through the motions.
I personally feel that 3-4 hours is a happy medium. It's small enough to be reasonably manageable, but long enough to get some very solid work done. I like to do it in two two-hour chunks.
Or you could try Sviatoslav Richter's recommendation: 3 hours.