Hello, just boat a 23' Starcraft Nexstar with a Merc Bravo 3 5.7 . The dealer just put in a new impeller for me before I took ownership. The dealer says it should be good for 4 or 5 years. Tha sounds to long to wait. Im running her in fresh water, in Canada for about 5 months a year. Any recommendations on how often I should change the impeller. Thanks...