To give an idea of the kind of sales volume a single trader can do without camping this is one days' worth of buying and selling on just one character. My Jita skill-book trader.
That's one out 12 active traders, with 6 more training up now and another 2 planned. And the 13th was a Monday, not a weekend.
I haven't done a full update of orders in over a week either. About all I've done on that character so far this month is post whatever was sitting in my inventory (meaning it was bought recently) and if necessary update/re-post buy orders for those items only (maybe 50 items at a time).
Edit: This character handles nothing but skill books and uses station buy orders only.