A few years ago I started replacing them after 2-3 years. My trailer tires usually failed on an interstate highway or, more commonly, a 2 lane high speed county road with no shoulder. IMO trailer tires are one of the least dependable components of a boat/trailer/vehicle rig so it's peace of mind to have newish tires driving back home from Chokoloskee at night after a long day.
I've worn out trailer tires -lots of them and popped more than my share, but I'm towing my skiff on the road well more than 20,000 miles a year...
What greatly shortens trailer tire life expectancy is either under inflation (it will say right on your tire what the capacity is - and what the tire pressure needs to be... mine says 1358lbs at 50 psi... run those tires at 30lbs and they'll get hot - too hot, all too quickly ). The other thing that gets them is uneven wear... Watch your tires like a hawk - particularly when the trailer is unloaded and sitting at a ramp somewhere - any sign of uneven wear means danger - the slightest bit of cord showing means you'd better replace that tire before it pops at highway speeds (and takes out your fender lights or occasionally even the entire fender...). Just another one of those "ask me how I know moments". Most simply run their tires until one pops - and at highway speeds there will be little left to find out what happened.... Yes, a road hazard can jump up and bite you, but under inflation or uneven tire wear is much more likely... Find a bad tire before it pops and you're money and time ahead in my book...
I actually keep two spares with me going down the road every day - for a single axle trailer (bad feeling, replacing a tire and realizing you still have 100 miles to go - with no spare remaining...). I long ago figured out that having a few extra spares ready to go at home (three of them with new tires mounted and balanced sitting in my garage)... When you're running constantly you're never around when tire stores are open (except for an outfit like Walmart...). With the extras I can wait to replace bad tires when I have two of them - another time savings for me...
Hope this helps - most of what little I know about trailering - was learned the hard way (and should have come with a box of bandaids....).
I think over inflation can be equally as bad. I keep seeing guys here and on THT saying they run the tires at their max psi no matter the load. With loads as light as ours that can cause issues too.
Example my tires are a max of 90psi, but if I inflate to that they round off and run/wear on the center of the tread only. At 78-80psi they wear more evenly, and feel better going down the road.
I had a problem with trailer tires on my CC boat several years ago. When I asked some tire guys about it they said boat trailers are notorious for this since most aren't balanced correctly. They weren't referring to the tires them selves not being balanced but how the boat is positioned on the trailer. They said my best bet was to get radials like the ones used on cars. Yes they cost more but since I didn't have to replace them every other year it saved me money in the long run.
Bearings are a different story, I replace those ever year regardless. It's also a good idea to keep a spare tire on a new hub so you are ready incase either fail.
I do a lot of towing with two boats and a camper. I replace tires after 3 years and the longest I would go is 4 years. After 4 years you are set for a blow-out on the road which is no fun. I keep a high end 12V charger in my truck to keep tire pressure at the correct level while on the road with multi-day trips. If I do a tow over 300 miles I will put grease in the Bearing Buddies before I launch the boat. I keep my grease gun in a Tupperware container to keep things clean.