Im goin to state the obvious just in case its useful and may explain why 2 riders have different exp with all balls-
Mud and water destroy bearings very quickly,if you regularly cross streams,or play in axle deep mud,expect to change often.also sand (which is often salty/corrosive as well as gritty)
you can carefully lift the new bearing seal and make sure theres plenty of grease in the new ones before installing.also grease the lips of the axle seals and the external spacers to help prevent water infiltrating axle assembly.i try and wheelie the front wheel over small crossings/mud holes as much as poss but the rear obviously has to plough through.
Good cond spacers/seals well greased are the first line of bearing protection and can be kept greased relatively easily
This
All balls bearings quality isn't even close to that of an industrial equivalent from a known brand - won't go through them, but a local industrial supplier will be able to recommend several which will hold up to the task much better.
I don't believe in removing shields/seals and repacking the bearing. A quality unit will already have the right amount in there to work at the specifications they have been built to.
What I do believe in, is forming a barrier to keep the water,dirt,etc away from the bearing shield. I do this by packing the area between the seal and bearing with a thick water proof grease. As part of the maintenance every time the wheel comes off, the dust seals get popped out, grease wiped away, and freshly repacked.
If replacing bearings do the seals as well. If the spacers are really grooving out consider replacing or aligning the seal to run on another part of the spacer. This gives 3 layers of protection before the water/dirt can actually get into the area where you don't want it.
I live and ride in the tropics, and even in the dry seasons every ride has some mud, creeks, water. Wet season see's the bike drenched for hours.
Avoid high pressure cleaners around seals and bearing housings. They only force things into places you don't want them.
So as above, there are many variables regarding how long a particular bearing lasts in a particular application. Using a quality product and applying principals/maintenance to keep that in its best condition will see you get the best life out of them.