In my own experience, I found that around 4 hours day would be the limit for actual reviewing, and even then that was on the higher end. I also had a load of other projects, reports to build, meetings, calibrations etc.
I found that after 4 hours of reviewing in one day, you really just start to burn-out or get 'grading fatigue' (although this can depend on the types of conversations you review, for example if its non-technical, or very short conversations, you may last a bit longer before the fatigue sets in)
There were of course somedays where I would not review at all due to other projects, and some days where all I did was review. But as a general rule, I wouldn't go over 4 hours per day!