Way too many. I use it to distract myself from school work at lot. Also I'm usually on when I'm watching something on Netflix, so I'm essentially doubling the use of my free time
I only come on ATRL when I'm bored. So I'm only on here for about 3 hours, (if I'm really bored) while I'm at school, a week. So I'd say that I spend no more than 6-9 hours a month.
The tab is open from when I get home until I go to bed (assuming I'm home all that time) but I only actually pay attention to it for about 3/4 hours a day, which is still too much considering most of that time is still just lurking.