When to replace tires on trailer?

I'll offer a counter argument to the others thus far and suggest that your 6-10 year replacement schedule is pretty acceptable. Assuming the tires are of good quality, with proper care(ie. Keep out of sunlight when not using, proper inflation, etc) that timetable is not unreasonable. We replaced our original goodyear marathons(made in Canada) after 12.5 years when I started to see cracks form. I know that is the exception and not the norm. We only had around 15k miles on them because we stay mostly in California and in addition to keeping them covered, I lifted my fifth onto jacks when it sat for more then 3 months. We replaced with the Maxxis 8008 last May. We've never once had any tire issues. No blowouts or nothing. Like someone earlier mentioned, I also know people who replaced after just 3-4 years to be proactive and had a blowout with the new set on their first trip. There's just no guarantee with tires. Some might suggest "if you love your family then you must replace every 3-5 years". Don't buy into this. That's what the petroleum/tire companies hope you to believe. If you've taken care of them and there are no cracks or visible dry rot, you might be able to get another 6-24 months out of them. It all comes down to your comfort level. Goodluck!