I changed out my plugs last week. They were a little over a year old, with 11,764 miles on them. When I installed those plugs, I used O-rings to help prevent water ingress into the plug wells. The process is described in this thread:
https://www.yamahasupertenere.com/index.php?threads/water-in-plug-wells-prevention-a-possible-solution.23850/page-3
I modified the method a little: in addition to the O-rings, I added a bead of Bel Ray waterproof grease on top the O-ring. This is how my plugs looked after a year of all weather riding:
The number 1 plug is the one on the downside of the bike when it's on the sidestand. This plug has been pretty rusted in the past when I changed it, that's why I did the O-ring method. This is how the coil sticks looked:
None of them had any corrosion on the metal sleeve.
Back when I did my first plug change, I numbered the wires for each coil stick from left to right, and numbered the tops of each coil stick from left to right with white out. It was just a way to visually double check the work so I didn't have one of those OCD moments of wondering if I hooked everything up correctly after I'd already reinstalled the airbox and fuel tank.