There is really no "average" amount of wear. I know guys that ride in salt water, in shallow, sandy conditions that have to replace their impellers and wear rings every 50 hours or so. I ride in a very clean fresh water lake and my impeller still looks like new at over 200 hours.
Usually the first sign that you have a problem is poor hole shot........the ski will feel like it is spinning out with no traction at first. Then it gradually picks up speed. If the hole shot is good, then I would not worry about a few nicks and dings. If you want to check to see if it is within spec. You can remove the intake grate and check with a feeler gauge. Or tape a gauge to a stick and check it that way. On your ski you want to see 0.45 mm or less of a gap between the impeller and the housing.
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100