I ride 6 hours/week continually eating the dust of the guy I'm following. (He's a nice enough guy but the trail is quite dusty.)
How often do I need to replace the air filter on my 2009 XT250?

I would check it monthly. A visual inspection should provide you with the info you need in order to decide whether or not it should be replaced.

Either lead or fall far enough back to miss the dust. Dust no bueno for machines or humans.

I'd check at 100 miles and more or less frequently depending on how it looks.

If I spend a day in the dust I clean the filter before I ride again no matter how it looks. Of course that's with washable foam filters which get cleaned no matter how much dust is trapped...with paper filters all you can do is check and guess.

For me a few hundred miles in the dust is enough to need to clean the air-filter because it's critical to engine life...but I think the human health issue is even more serious and you should never "eat dust". I would insist on a riding system or ride alone.

Spacing is the key and I agree with onceana...there is no advantage or reason for wheel to wheel trail riding on a dusty track.

Before you ride set a policy that the lead rider will stop and allow a pass after a certain amount of time, say 15 minutes or so, and then resume the speed after the following riders have past and the dust has settled. If there's no breeze, a few minutes or so spacing between riders works good.

The rule for my group is if you are leading and you can't see the following rider DON'T slow down until he "catches up" and then take off again. Let him go by and take your turn.

I know a rider who goes like a bullet making clouds of dust.
If you slow to leave a space for his dust to settle, he slows too until you catch up, then away he goes again. This almost always results in a race for who gets to ride in clean air.

I would set some rules or ride alone.

patuca

We usually try to offset our bikes. That works depending on wind and dust levels. The engine is one thing but your lungs are more important. WE ALWAYS wear dust masks. Here the county roads get mag chloride treatments. Can't imagine breathing that dust under any circumstances.

Is the stock XT250 air filter a throw-away design? Thought it was a serviceable foam filter like the XT225. If it's the latter I'd probably clean it weekly; it really doesn't take long for the air flow to get restricted under dusty conditions. Buying a second foam filter will solve the issue of service downtime, ie. waiting for a recently cleaned filter to completely dry before re-oiling. I guess avoiding water-based cleaners could shorten the downtime as well.

Agreed on the dust. Either ride close enough that the dust isn't flying high, which takes a lot of trust in your companion, or stay far enough back that it settles down or blows away. No reason to ride in the cloud and suck up all that dust, either you or the bike.

I've also ridden with guys who like to "roost it up" and threw up a huge cloud of dust. After the 3rd or 4th time they spent 10 minutes or more waiting for a few of us to catch up, they finally got the idea that the whole ride went a lot faster if they quit intentionally kicking up tons of dust.