carrying a spare bearing (and the tools to change it along with some grease) is a great idea.
I'd note that these trailers aren't boat trailers that get submerged every so often. I'd check/grease/replace the bearings on any used trailer I bought as well as regreasing the bearings on a new trailer but these bearings SHOULD last a long time.
a bit of an "apples and oranges" story but I had gotten used to replacing the bearings on my race motorcycles once a season and so, when I transitioned into streetbikes, I kept doing the same thing (regreasing or replacing once a year) until a fellow rider with the same bike (an engineer who puts on lots of miles on his commute) tracked the replacement of various parts......he got 96,000 miles out of his wheel bearings and they were still serviceable at that time! Needless to say, I've pulled back on my maintenance schedule for these. ( I have multiple bikes so I put less than 5,000 miles a year on each one)
I'm not one to skimp on maintenance and longevity will depend on the quality of the bearings, amount of grease and proper installation/preload, etc. - what I will say is that a good quality set of trailer bearings, installed properly and with adequate grease should last for many years (depending on usage, of course). Other than time for the DIY guy or a bit of $$ to get a shop to do it, there is no downside to "shorter than necessary" maintenance intervals. I would also guess that the majority of TT's never see any wheel bearing maintenance unless/until there is a problem and that some have gone decades without attention....and NO, I don't recommend that!