When I was grading for a 75 student class (one section) I'd try to spend 10 minutes per midterm - 15, max. At 10 minutes per midterm, that's 750 minutes, or 12.5 hours. At 15 minutes, that's something like 19 hours. If the teacher has two midterms and a final exam, that's anywhere from 37.5 hours to 57 hours. The year before I started grading, students were getting paid $400 per class to grade. When I started it was $300 per class. Last year it was $200 per class. I didn't grade last year. While I'm glad I had some grading experience, it wasn't worth the time it took from my studies. Some students are so poor and desperate that they lobby to get three sections to grade.

Here's the breakdown. This is in San Francisco, where the minimum wage is around $10 an hour. Let's just say after grading papers I really have come to value the comments my instructors left on mine.

$400/37.5 = $10.66 an hour. (10 minutes per midterm)
$400/57 = $7.02 an hour (15 minutes per midterm)

$300/37.5 = $8 an hour
$300/300 = $5.26 an hour

$200/37.5 = $5.33 an hour
$200/57 = $3.51 an hour