Glad it helped you!
Hate to sound like a broken record...or is that "DVD" these days
It really depends on how the trailer is stored, where it is stored, how often used, and conditions it is used in...
Brand new springs on a brand new trailer have a very thin layer of really cheap paint that seems to invite rust quickly.
The outside (the part you see) looks okay for some time, but the first time you pull the trailer, the springs flex, rubbing off the paint, allowing rust to begin forming where it can't be seen.
I'm of the opinion that if the trailer will be kept for a while and used regularly, the service should be done when you buy it, new, or used.
The greasable bolts should be greased each year, or every couple thousand miles, and any pivot points not greased should be sprayed with grease or heavy oil.
Taking the assemblies apart for cleaning and re greasing would depend on usage, but for the average camper in the rust belt, maybe every 10 years, assuming you spray lithium grease, or some other good sticky grease against the sides/in-between the leaf springs yearly.
If a person is really religious about it, you may never need to take the springs apart in your lifetime.
Maintenance; do it regularly, or pay big later...