Are you not running an air filter?
I have had 6 KX250F's (2004 (2), 2006, 2007, 2009, 2011) that were raced on a regular basis. All of these were run for 90 hours without a single valve adjustment.
Here's what I do/don't do.
Always be VERY careful in regard to dirt. Never let dirt into intake tract when changing filters. Never leave gas cap open at dusty track. Don't use a gas can with no cap. Never work on bike dirty. Maintain bike at home, when clean, in clean shop, not at the track. Be meticulous.
Never start bike cold and immediately rev it up. 80% of engine damage occurs during cold start-up/revving. Don't "test rev-limiter" while at starting line because the idiot next to you is doing it. Don't ride on rev-limiter during race, in an attempt to sound like Barcia. There is no traction in the air. The bike makes peak power well-below redline anyway. It's not a 2-stroke, don't ride it like one.
Change oil every 7 hours, filter every 14. More is not better, just wasteful.
Some of these may seem obvious but are based on what I see at the track every weekend. I have been racing since 1975 and this works for me. Contrary to popular opinion, I believe a properly cared for 4 stroke is less expensive to run than a 2-stroke.