It's recommended to repack the trailer wheel bearings with new grease every 10,000 to 15,000 which is usually once ever year or once every 2 years or so depending on how many miles you put on the trailer.
If you do a lot of towing of the trailer and put a lot of miles on the trailer wheel bearings then you may need to repack the trailer wheel bearings once per year.
Then if you don't use the trailer all that often you may only need to repack the trailer wheel bearings every 2 to 5 years.
But every few years it's a good idea to remove the cap from wheel bearing hub and check and see if it's properly greased and has no debris in it.
I would also recommend jacking the trailer up and spinning the wheels and listen for any grinding sounds or move the trailer wheel back and forth to check for any play.
That would let you find a problem so you can fix it before it causes problems on the road.