Our last trailer was new. We had a bearing failure in the middle of nowhere in Arizona. At that point, the trailer had about 3k miles on it. It cost nearly $400 to have it repaired roadside (single axle, I was stranded). We got the other side repacked when we got to the next town. I watched the mechanic pull it apart...it was BONE dry.
My current trailer has about 2k miles on it now, and I keep putting it off...I guess I have to learn the really, really hard way. :) We leave for a 6k mile trip in June. Best believe they will all be repacked before then.