When to change Dry Joint Manifolds.

Hi everyone...I'm new here.
I have a 2011 2855 MAG350 MP Bravo III.
It was a left over from 2011, so I started using it in the summer of 2012.
I put approx 30-40 hours per year on the engine.
Use it in Brackish water.
Raw water system
Manifolds are Dry Joint.
Two questions here:
1) When will it be time to change the Dry Joint manifolds/risers?
2) When I use my IF temperature gun, what temps will be normal at my normal cruise with 4 people @ 4000 RPM's?

My temperature gauge is a steady 180. It has been that way since I purchased the boat. I had changed the thermostat and there is no change. When I use the IF gun on the thermostat housing, it shows approx 160-175 depending where I aim it.

Thank you!
Mike

The smart a$$ answer... The day before they fail!
I just replaced the dry joints on my late season 2006 260HP 5.7MIE with "fresh water" cooling. Boat used exclusively in salt water. The engine had about 230 hrs on it when I changed out the elbows. I am in the habit of flushing the system several times a season ( May to Election Day) with fresh water, and at least once a year ( in the fall) with Salt-Away. When I removed the elbows, they looked as if they could easily go another couple of seasons.

That said... " Cruising @ 4000 rpms" a bit high. For best engine life you should limit cruise to no more than 80% of max wide open RPMS, i.e. if the max RPMs you get with normal load and full tank is 4400 RPMS, you should cruise at no more than 3520 RPMs, 3300 (75%) would be better.

Engine temps seem a bit high. Normally, 140ish T'stat for raw water cooled engines in salt water, 160ish degree if in fresh water or with engine with heat exchanger (i.e. FWC)... In brackish???? If it were my boat, I'd consider a 140 degree T'stat, but then again, I'm a semi retired R&D engineer and a "belt and suspenders" guy. It's your boat, you pays your money, you takes your chances...L.O.L.