I'm a semi-pro and have never sent in my 2 year old 70-200/2.8II. I haven't heard of such a practice. I would think more like every 5 or 10 years??
Same here. I never send a lens in unless it needs some type of repair. My 400/2.8IS has gone in for bearing repairs but my 300/2.8IS is over 7 years old and has never needed repair.
Even though my gear gets used outdoors 99% of the time, it never gets so dirty that I cannot clean it myself. I'm from the "If it ain't broke don't fix it" school.