How often do you replace the aforementioned items? I just bought a used boat and plan on replacing the impeller. I'm considering having the lower unit seals replaced too even though I don't have any noticeable problems in the seals as of right now. Typically I'd say if it isn't broke, dont fix it, but id like to see how often , if at all, you all replace those parts

your going to get a lot of people saying to replace them once a year or so.  I replace my impeller when no water comes out of the pee hole.

 

But my boat is only a 1985 so maybe after a few more years the neglect will ruin it

 

just kidding, its a joke.

Water pump impellers I replace every three to four years.  By then they have a started to harden and memory set into them, increasing the chance of failure and reduced volume.

 

Change LU lube every fall, before the temps start dropping into the freeze range.  Doing that, if it has developed a leak and gets water in it you don't know about, it's not going to freeze and bust the housing, and the lube should be changed each year.  As for replacing the seals, leave them alone, it's too expensive and the chance of them leaking is slim, unless line gets wrapped around the shaft and cuts one out.

Under warranty, impellers need to be changed annually.  After that- all but one boat mechanic I ever asked said 2-3 years, unless there is a problem (obviously).

On a used boat, I'd change it immediately. Every 2 years is my schedule, with heavy use. Since I run a Merc Racing outboard, I don't wait for water pressure to drop to react.