I see very cheap (under $20 new) pneumatic pumps driven off
of shop air, about 4 CFM air use. Has anyone here have experience
using one of these? Do they pull a good vacuum ( I have seen
notations of drawing 27 inches Hg of vacuum).

That will do just fine, if you add a refrigerant backfill to "rinse" the
system and repeat the rough vacuum. With a pure backfill gas and enough
repeititions, your rough vacuum is as good as a hard vacuum at removing
non-condensibles, which is the point. Indeed, with rinsing you don't
even need a vacuum; the vacuum just decreases the number of rinses
needed to achieve a target purity inside the system. Backfilling and
rinsing is not strictly legal in some jurisdictions, but it's no
different from shooting a few ounces into a leaky system to test for
leaks.

There's a superstition amongst A/C techs that pulling a 50 micron vacuum
is needed to "boil off" moisture from the dessicant. In fact, no vacuum
will regenerate the dessicant. If the system has been contaminated, you
should replace the dessicant.

If you look at the OEM factory manuals for auto A/C back in the 1970s,
you'll see that before the recycling machines came into use for the
Clean Air Act in 1990s, that a very crude vacuum pump is what
*everybody* used. The authorized GM A/C vacuum tool from those days was
a small piston refrigeration compressor unit.

This sort of superstition is encouraged because the pros believe and
sell the idea that only they have the tools to do the job, which no
DIYer could reasonably afford. It also justifies the $5K they were
suckered into paying for a recycling machine.

Oh, for those days when you fixed your auto A/C hoses with tire patches
and hose clamps, went to K-Mart and bought an armful of R12 cans for
$1/each.

The Harbor Freight gadget is very inefficient and takes a lot of air
compressor CFM to run properly, more than most homeowner compressors can
deliver, but given that, it does work for a rough vacuum, plenty for
improvising A/C work.

How long does one need to run such a vac pump to draw a good vacuum?

Again this is typically governed by superstition. The point of the
vacuum is to remove non-condensible gases. Whatever vacuum you have
will do whatever it will do in a few minutes. But you'll find techs who
leave their $500 rotary-vane pump on there overnight, thinking it
somehow must be better.

Your effort is best spent on fixing leaks with a vacuum test. It is all
a waste of time if your system leaks in the slightest.

I have studied and worked on dozens of my own AC systems for 35 years,
and proved these improvised techniques many times.