I think it varies tremendously.
I’m on here about 40-60 hrs/week.
Marley would be the outlier on the high side. He’s on here 100+ hrs/week. 
I still have my coffee mug, too.
It varies not only from mod to mod but by mod, depending on what we have going on elsewhere in our lives. I’m a freelancer, for instance, so if I have a deadline on a given day I’ll give my fellow mods the heads up that I’m not available. Other days I’m mostly responding to post reports; still other days I’m reading not only “my” forums but the others in a leisurely thread-by-thread way and either dealing preemptively with stuff or flagging posts in forums I don’t mod.
Some of the older columns occasionally had a section called “Questions we’re still thinking about” which I think is what these were. Haven’t seen it for a while.