I probably have 20000 miles (mostly interstate, at 60-65 mph) on my 3 year old Goodyear Marathons, and hope they give me one more year and then I'll replace them.
I had them trued and the running gear balanced when we first bought the trailer, I put 303 UV Protectant on the tires several times during the year, and store the trailer inside much of the time from mid-October to early June. I also always keep them fully inflated at 65psi.
My tires look practically brand new, and who knows they might give me another couple years of use, but I'm not going to be penny wise and pound foolish. If I stored my trailer outside year round, towed more on rough roads, etc., I would have consided putting new ones on sooner.