Impellers

[Eddies_Boat]

Hello!
How often should I replace the impellers on my 1999 Express Cruiser 215 with a 5.0 Mercruiser stern drive?  Every 2 years?  Every year?  Every 100 hours?  
Anyy comments/suggestions would be appreciated!
Thanks!

[Jeff007]

My shop says every 5 years minimum for freshwater use. I replace sooner when I suspect outdrive has been in sand.