In addition to personal preference, it also depends on the type of wood you use and the meat you are cooking. It seems like leaner meats tend to absorb more quickly and are easier to oversmoke. With a pork tenderloin or salmon, 1 or 2 chunks seems to impart plenty of smokiness for me. On the other hand, fatter meats (ribs, pork shoulder, etc) seem to be more forgiving and can take a lot more smoke. For example, in my experience, 3 hours is plenty for a pork shoulder. Brisket has plenty of fat, but is more spongy, so I would use a little less.
As far as wood is concerned, it's a lot easier to go overboard with mesquite. To a lesser extent, same with hickory and oak. Apple, cherry, maple, and pecan can go further before they overpower the natural flavor of the meat.
I hope this helps.