Guess I will jump in. Sounds like you have the 4 cylinder Mercruiser, which has an Alpha 1 outdrive.
You should probably change the impeller every other year or more, depending on usage. Even if it is not used much, the impeller fins can "take a set" and not pump effectively or become brittle so the first little pebble might break one off.
You will have some friends tell you they have the run the same impeller for years with no problems. IMHO, they are living on borrowed time. A $20-$50 impeller even each year is much cheaper then if you overheat the engine. Its like my buddies that like to downshift a manual trans coming to a stop in their cars. They say they are saving the brakes. To me, a $25 set of brake pads is cheaper then a clutch and tranny rebuild, but I digress.
Here are my usual boating forums, in order of how I like them.
http://www.goboating.com/main/
http://www.boatus.com
http://www.boattest.com/main.asp
http://www.boatered.com/forum/
http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi
http://www.usboating.com/
My Alpha 1 is still new, but I believe the impeller changes like my old outboard did. You need to put the drive selector in forward, and drop the lower unit. The impeller should be on top under a little cover.
BTW, $125 to do that, if it includes parts, is really cheap. Most marinas are around $250 for an impeller change.