I think it depends on how the tires have been used. If the trailer was used monthly I think you can go longer than 7 years. Rolling the rig keeps the lubricating oils in the rubber out near the surface preventing cracking. I would have no problem going 10 years if used regularly.
East of Casa Grande, AZ in the town of Coolidge is the Casa Grande Ruins National Monument. Stop there and take the free tour. It is a very unusual prehistoric ruins and the only one of it's kind still standing.