I'm nearing about 50 hours per week. Without work etc, I can do about 6-7 hours per day. I time myself though, I'll have a 30 minute countdown, then after the times up I'll see if I'm up to another 30 minutes, if not I'll take a small break then just repeat the process. Hence although I'm at my desk for at least 12 hours per day now, I split my time accordingly. Need to sleep more though, I'm only getting about 6 hours on average per night.