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)