This may not be important, but it was something that I would find pretty neat to know. And the question is simple, how 'deep' will a given grit (36,80,120,etc) tear into the wood. I do not think there is much more to be said.
You can see the diameter of the average grit in a piece of sandpaper here: https://www.gessweincanada.com/category-s/11328.htm
Assuming the grit is embedded at least 1/3 of this diameter, you would "cut" into the wood about that far per row of "grit". So it depends on the length of your sandpaper (a 1 inch piece of 80 grit sandpaper is going to cut less than a 10 inch piece per stroke, probably about 10:1 if I had to guess).
But really sandpaper does not "cut", it more tears or smashes the wood fibers.
This is a "How long is a piece of string?" question.
Lots of factors:
- What type of sandpaper is being used (open coat or closed, abrasive type* etc. etc.)?
- How hard is the sandpaper being pressed into the wood?
- How fast is the sandpaper moving?
- A key factor obviously, how hard is the wood?
- And last but by no means least, how long are you sanding for?
In theory there's no limit to the depth you can sand to, with any grit within reason, but obviously in practice there are limits in terms of cost (paper wears out, or fills or corns develop), user patience (who hasn't gotten bored sanding?) and the time available (sanding is rarely the fastest way to do something in most home woodworking environments).
With powered sanders what's feasible to do with sandpaper have opened up enormously — just look at the type of stock removal someone with a belt or disk sander is comfortable doing versus someone who has to sand by hand — but the traditional way(s) the same task was done are worth investigating and trying out for oneself..... bearing in mind that in time almost everyone seeks to reduce the amount of sanding they do to a minimum.
*Different abrasives have different hardnesses and the individual grains are different shapes, some with sharper edges than others, and each type wears down in its own way. So some papers initially cut fast but quickly lose efficiency, others can be slower to begin with but the falloff in sanding efficiency can be much slower.
What exactly do you mean by "penetrate"? Since the sandpaper will rip small particles from the wood you can actually cut into wood as deep as you want as long as the sandpaper is still sharp and the sanding dust does not block the sandpaper.
For example you will remove top one or two millimeters from a parquet floor using a belt sander to remove the varnish and small dents and scratches in the wood. This is done using a sandpaper belt which surely is thinner than those two millimeters.