Several things tell you it's time to replace the impeller in your outboard motor. One is in print; the rest you will see if you only pay attention while operating the motor. Ignore the symptoms of an impeller in need of replacement only if you wish to purchase a new motor.
The Maintenance Schedule
The maintenance schedule for your outboard is located in the final pages of your motor operator's manual. All outboard manufacturers recommend that you at least inspect the impeller every 100 operating hours or once yearly, whichever comes first. As an astute boat owner, it's certain you'll realize you might as well replace the impeller -- slide it off the driveshaft and slide a new one on -- since you've already removed the lower unit and the pump housing cover to make the inspection.
When the Outboard Shows Signs of Overheating
Anytime your outboard motor's water temperature begins to climb, that's cause for concern. If the motor ceases to produce a steady stream of water after reaching its normal operating temperature, the impeller is one of the two most likely causes. If you just took your motor out of extended or winter storage, check the cooling water outlet located at the bottom rear of the motor's top cowling: All across the U.S., wasps like to winter in there for some reason. The other most likely cause is a damaged or worn impeller.
A Reduced Stream of Water at the Cooling Water Outlet
If you see a smaller-than-normal stream of water coming from the cooling water outlet, it may be wasps if this is your first outing of the season. A thin wire inserted in the outlet will confirm or rule out that diagnosis. If this isn't your first "run" of the boating season, though, the most likely cause is a damaged pump housing -- like a hole in any water pump, it causes a pressure drop -- or a worn or damaged impeller.
The Impeller Has Been In Service for Three Years
If you haven't changed the impeller when you inspect the water pump, as part of the preparations for the winter layup or the spring fitting-out, three years is the outer limit. One manufacturer's yearly or 100-hour maintenance schedule does not include an inspection of the water pump and impeller, but insists you change the impeller at 300 operating hours or three years, whichever comes first. Change the impeller. It's one way to come closer to your boat and motor.