If you just bought a rig would be a good idea to just change it. That gives you a baseline. After that it just depends...................If you have trouble sleeping at night maybe every season. If you have been around boats all your life and don't run in damaging environments or leave it sit up immovable for long periods of time, then whenever you feel like it in my opinion.
My last boat was after 10 years of my ownership because of clean operational environments and watching the symptoms, I wasn't concerned....course I didn't go off shore, operating in salt/sandy/silty water either. I just figured I'd change it one day when I felt like it. When I removed it, it was intact but did have a "set on the last ⅓ of the blades which is what happens with age.....set being the blades are bent back toward the ends rather than being straight out (like wagonwheel spokes) when new.
However, I had sensors to warn me if I had a problem and my "post flights" would have given me an indication that it was working fine. Back before I joined iboats I don't ever recall changing an impeller on an outboard...always bought new and some, other than the last one which was used (2 years old and I had a box of old parts where the PO had just changed it and I kept for 12 years}, were 7ish years old when I sold them, both OMC and Mercury brands. On here over the years, the replies to that question were A to Z
While you are at changing rubber, don't discount your fuel supply lines. At 10 years mine started to dissolve and pieces would up in the fuel flow channels and caused problems. I'm not going to get into a fuel squabble over the whys but fact is they did for whatever reason......never had hose problems over the years....hint hint.
