I will go with every 4 years, less if they look bad.
I had Nanco radials on the Wildcat from factory, and they went 4 years with no issues. I am also very diligent with air pressure and weights on them. I also balance all my trailer tires. It does make a difference.
Last year I went to Maxxis E range tires. After a few long trips, there is a noticeable ride difference....much smoother. I do also run a TST tire moniter system.
Steve and Vicki
2011 Dodge 3500 CTD Outdoorsman
2013 Columbus 320RS-ITS HERE!!