Yeah, but.... Hold a new filter up to a light and you can see light
thru the filter, similar to a lamp shade. That same filter after
being blown out 2-3x and as clean as you can get it, will not let any
light thru. This tells me dust is imbedded in the pores of the
filter. Obviously air is getting thru it somehow, but if the airflow
is impeded, is it pulling unfiltered air from around the edges? And
if the air/fuel ratio is out of whack, is there unburned fuel deposit
buildup happening in the engine? I don't know the answer to these
questions and that's why I asked.

OK, if the air filter is extremely clogged, that could reduce airflow
through the engine, causing the engine to run a bit rich. In a car or
motorcycle, this can reduce fuel economy. For example, a car that gets
25MPG highway might get 23MPG highway with a clogged air filter. The effect
on a lawn tractor? Probably similar, but a lot harder to measure. I mean,
there's no odometer on a lawn tractor. So how do you really keep track of
how much fuel you are wasting with a clogged air filter?

There will always be some small amount of air that gets pulled around the
edges of the air filter. Even a brand new air filter. The air filter isn't
meant to stop ALL dust from getting into the engine. It just isn't that
critical. With no dust in the engine, the engine would still wear out
eventually. And with a little dust in the engine, the engine will still
likely last longer than you need it to, if you don't abuse it. (in
particular, don't neglect to change the oil regularly)

There is no specific answer to your question, simply stated how often do I
need to change the air filter? What it boils down to is a judgement call.
I'd suggest that you change the filter once a year. Change it more often IF
you happen to damage it while blowing it out. Keep a spare on hand just in
case, as when you find you need one, it might not be a convenient time to go
out and get one.

Also pay attention to how the tractor is performing. Maybe you don't have
an odometer to measure fuel economy, but you will spot other problems if the
fuel/air mixture is too rich. For example, if you hit really tall grass
with a new air filter, does the engine bog down / lug? After you blow out
the air filter a few times, hit a similar patch of really tall grass, does
the engine STALL now? That could be a clue that your engine is not
producing as much torque as it should. Possibly time to change the air
filter? Although that could be caused by other things, also. Also, if you
normally mow the whole property on a single tank of fuel, but after blowing
out the air filter a few times you find that you need to refuel when the
mowing is almost done? That could be another clue that the air filter is
due to be changed.

Just use common sense. It's an internal combustion engine. You don't need
to be a rocket scientist to operate and maintain it. -Dave