Cleaned the stack about 20,000 miles ago (about 1 year). It wasn't a full pull out the radiator cleaning, but I did get in there. I plan on doing a full tare-down cleaning soon, but I just cannot imaging that it got that dirty in 5 months.
I didn't install them myself, so I am wondering if they maybe installed them 180* rotated with the holes in the wrong direction. Maybe they accidentally flipped the 180* and 190* thermostats. I don't know...