Generally people tend to feel overwhelmed by more than 250 reviews a day. A lot of people consider anything more than 150 reviews a day too much. The nice thing about WK pacing though, is that doing reviews without doing any lessons for a few days will make the workload easier within a week, and it’s simple enough to decide to not do as many lessons when you feel you can’t handle as many reviews as you used to .
I don’t have ant hard stats, but I think that the master, enlightened and burn reviews add about 20% reviews each to your workload. In addition they feel harder than regular reviews (especially burn reviews, but enlightened reviews also suffer from that), and that can demotivate people.