While I'm of the opine that the rubber or syn rubber that the impellers are made of can last through many, many seasons [I've seen known 7-year old ones still look great!], this truly is the #1 service item that is strictly PREVENTIVE maintenance at its best!
I'd opine, depending on use, have it done with FULL waterpump service including the thermostats [NEVEr just replace the impeller! IMHO], every 2-years ... dropping it to annual basis if you run in heavy falts, sand and/or silty conditions and do mega hours each year, and maybe push it out to 3-years if your annual use isn't too much.
To me, the biggest reason to do so is to ENSURE one can take the lower unit and all bolts to-be-removed APART ... as if/when you get a frozen bolt ... you just doubled or tripled :shock: the co$t of your service bill! Clean ALL of the bolts and dip into gasket sealing compound before re-installation and torqueing.
Buy the OEM factory service manual, a torque wrench, and the needed grease and lubes, all for ~$120 or so ... and then do it for the price of the rebuild kit itself ($60 to more, we know Yam gouges for parts co$ts). FWIW, I can do the full service on a V6 Ob is less than an hour myself, though it does help to have a 2nd person for the re-install, due to lining up the water tubes et al, while holding the heavy LU.
