Yes, 10 hrs a month is the requirment for a MS/Elder but not documented. It probably is pencilled in the Elder's book. It's an unwritten rule...just like the beard thing. No where would you read that you don't appoint a brother who has a beard.
It now goes further in that the wife, and children must have close to that average. I remember when I served as a Elder and going through the questions for baptism, the CO chastised me for allowing a young person to be baptised who didn't get 10 hrs a month. He sure was a CO from Hell.
Now the wording they use in the publications is, "Must be visible, in service", or " Meaningful share in FS". That means out every week, at least the saturdays, and some Sundays.....do the math.