If you feel you can't sharpen them well, then send them to be done professionally. Keep the boxes for the postage.
Most japanese knives are very hard and if you care for them, you probably won't need them sharpened more than every 6 months at most.
You can easily strop them using any thick piece of smooth leather, that will keep them tip top.
I recently repaired a global knife with exactly the same kind of nicks. Too small to feel with your fingernail, but definitely felt when cutting paper.
You'll need to re-profile the knife by using a coarse stone (even a brick will do). By holding the knife side perpendicular to the stone surface, and drawing from heel to tip, until the knicks are gone. This ...
In terms of time, it depends on many factors:
the degree of existing
So we can't give you an exact answer.
However, the answer is when there is a burr.
Its haRd to feel a burr even with finger tips, so you can use the fingernail test. Here is a good guide.
It does take roughly that time (45m) to sharpen a knife with a sequence of whetstones.
However, you should not need to do it often. With a 60+ hardness, just some gentle stropping once a week is enough to maintain a sharp edge for at least 6 months. Stropping takes a few minutes.
You will only need to sharpen after lots of heavy use, or if it has been ...
There is such a report, but you're not going to like the result. According to KnifeGrinder's scientific test(PDF), plastic board materials other than low-density polypropylene are the least edge-wearing board materials you can get. Wood wears the edge more than plastic, as a rule.
On a whetstone for use with water you use a nagura stone.
There are natural and artificial nagura, cheap ones and expensive ones.
The nagura is harder than your whetstone.
You rub it on your whetstone in between sharpening your knife whenever you feel the surface of your whetstone is loaded (clogged) with particles.
It abrases the surface of your stone, thus ...
I found https://youtu.be/kwg18GUJ3nw?t=222 where he explains that angle guides are nice, but they can reinforce bad practice, take away your ability to sharpen by feel, or be flexible with regards to the angle at the tip. Moreover, the clip-on guides can scratch your knife, which is sometimes undesirable if it has a nice finish (e.g. Damascus or hammered).