When basic maintenance for basic Suntour XCM/ XCC fork is needed? When basic maintenance for very basic Suntour XCM/ XCC fork is needed?
Is it needed just in case if something wrong or once a year or after some amount of km?
By maintenance, I mean something like this:
According to SR Suntour's general fork manual (page 14), there are two maintenance intervals:

every 50 hours of riding: check torque on the bolts, check for stanchion damage, oil leaks, etc.
every 100 hours of riding: 50-hr maintence + disassembling the lowers, cleaning the internals, replacing oil, replacing seals and slider sleeves, etc.

In my quick web search to find in the owner's manual, I noticed some links where this fork was for sale for less than $75 (US).
Now the last thing I want to do here is bike shame anyone.
But in my experience, forks such as these are more weight, maintenance, and cost burden than they are worth in performance benefits.
This is because (just a guess) the seals, sleeves, and lubricant will cost you about $15 - $20 and the shop labor would cost around $40 - $50 to perform the 100-hr maintenance.
Their low performance comes from their weight, low-quality (high-friction &amp; easily damaged) sliding surfaces, poor seals that leak oil and permit contaminants to damage the stanchions, poor damping, lack of spring rate adjustment, and lack of damping/rebound speed adjustment.
Selectively reading between the lines a bit, if you're not comfortable paying essentially the price of a replacement fork every 100 hrs of riding or sinking your own time into it, I would recommend finding a good rigid fork to pair with a high volume &amp; supple tire.