Personally, I change mine every year just to be safe even though I fish a lot. It's actually harder on the impeller if it sits all the time.

the water pressure guage is your friend,
I have changed impellers in less than a year, and one (a good one) went four years...usually get 4-5 years on the WP housing
you will probably get better answers in the "boats and motors section.

I agree, pay attention to your water pressure at idle, low speed, mid speed and higher. If you see something that is different its time to inspect and probably replace.

What is average cost to do this? I figure both water pump and lower unit gear oil? 225 opti...

If you fish alot and still have good pressure every 2 yrs is fine on that motor. It has a bigger impeller than older mercs. If your boat sits alot or you run in alot of shallow water then every year. I would have it done by someone who can also pressure check L/U. I never had this done til yesterday and am so glad i did. My inner prop shaft seal was leaking during test and was about to go out, not good but i caught in time. Knuckheads who assembled it obviously didnt use spline cover when they put carrier bearing together causing inner seal to fold over on shaft.

+1 on once a year impeller change. How often it may need changed depends on how shallow water you are running, stirring up mud. Also loading the boat at some ramps isn't the best.

I charge $127.50+parts normally about $190+/- parts labor and tax depending on your motor

Had mine replaced last month. There was a lot of sand buildup in my 200 Opti. Put it in the lake to run it and my pressure was 15 at idle where it was running 4ish before.

So whatever you have on a new install let that be a baseline or a standard so over time when you see it drop then you know its time for another.

Every year! you would be surprised how many little things you will find that will save you a grenaded lower unit. Line around pro shaft. Flow tourq hub with slop. Transom bolts loose. Just to name a few.

5 yrs. You will get a hot warning before you tear anything up. Keep an eye on your stream.

My Optimax would shut down if it started to overheat, so don't worry about a meltdown. However, it would not be fun if you got in some bad weather over on the Mexico side and your impeller went out. In this case I would change it out yearly, otherwise every 2 years. You can still tear up a new impeller if you get sand or debri in your water pump.

EVERY YEAR, JUST HAD FRIENDS BUZZING AT US ON POOLE LAST WEEK, HIS CHANGED EVERY YEAR AND STILL SCREAMED AT US.MEAN OLD MERCURY. BUT THEY ARE GREAT ON FUEL