I did proper assembly from the start where I cleaned and greased them when they were brand new. I've never had any issues since. No (cheap) linear bearings come with grease. They only have assembly oil and that is not even a load bearing lubricant; it is a corrosion inhibitor. This is a good thing really, because you need to know exactly what grease your bearings contain. You should never mix greases of any kind. They all have different formulations and will act unpredictably when mixed; often failing in a coagulant that provides no protection from metal on metal contact.

Many cheap printer manufacturers will dab a bit of grease on the rails outside of the bearings when new. This is useless in practice due to the bearing seals. The seals are designed to let a small amount of grease out, but block any old grease from reentering the block itself.

If the blocks were run dry without grease, they are contaminated and need to be cleaned out completely. Likewise if they need service and have unknown grease inside them. If you clean them out to the point they are spotless, and then you manually pack them with a quality grease, you're unlikely to ever need to service them again for a very long time.

I build my own bicycle wheels and service my bearings and hubs about every 10k miles riding in all weather. I was sloppy with how I serviced bearings for a few years before I really narrowed in on my issues. They must be spotlessly cleaned, without any old grease whatsoever; like clean enough to eat off of them. This is the difference between 2k-4k between problems and 10k+ on a daily ridden bike. Same thing applies here if you want to only do the job once.

Currently my mk4 is printing pretty much 24/7 with IS profiles. I'm applying some lubricant roughly once per week - sometimes I notice the printer starts making strange noises, mostly I notice when rods have zero residue between prints, and just add a bit.

If I'm honest? probably nowhere near enough.

The 46 hours is assuming it moves at 300mm/s on that axis for 46 hours, which just isn't the case.
I say this, but as a ballpark figure this is still useful. Even if typical prints probably take longer than that to reach 50km on an axis, that still tells me I certainly don't lubricate them as often as I should.

Maybe that's something printer firmwares could one day be modified to calculate and warn the user about.

An average speed of 300mm per second on every axis? Including acceleration and cornering? How fast are you printing? 1000mm/s?

You might be a factor of 10 off here…

Generally lubricate every few hundred hours, however you really should consider condition based lubrication over time based, over lubrication is actually a really common failure mode for rotary bearings and would not be surprised if the same is true for linear bearings. 50km intervals sounds like an order of magnitude too frequently based on what I recall from Thompson, yeah the bearing type, loading, cleanliness etc all play a part, but their examples are in the hundred of km range, not tens.

Are you ensuring that you're actually getting grease into the bearing as well? The MGNxxX bearings are usually sealed, you need to get past them to actually lubricate as you want to expel the degraded grease. Myself, I do some white lithium via syringe and then machine oil on the rails, and even that's probably excessive. Moving the bearings by hand along their length of travel will give you a feel for them as well, there's a lot more you could do but I'll be totally honest that it's probably not worth doing, consequence of failure is basically nothing in the hobby space (no risk of injury, low costs, no impacts to business, basically if a bearing goes you're out what like $50? and an hour)

Wouldn't you like to know 😉

My Ender 5 Pro has been running for 6 years without any maintenance at all. I've leveled the bed probably 8 times in total. My prints all come out great.

It's not so much about lubrication as it is about trying to prevent seal wear. When the wiper seal gets worn and can no longer keep the dust and grit out, the rolling elements have little time left.

If the bearing as lubricated correctly at installation time, it should never need re-lubrication. But it's important to keep the rail surfaces clean. And it's amazing just how dusty those rails can get and just how abrasive that dust is on the bearings.

I don't lube the rails, it attracts too much dust. But I do wiper the rails periodically. I replaced the linear rod bearings and rods on my old and trusty Mk3s+ after almost 3500 hours of operation this last winter. Though I should have probably done it a bit sooner.