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If you don't want to replace it that often you can vacuum it out regularly. This will get out a lot of the dirt and the larger things that get trapped in there.
Also, not sure what year your car is but on my 2007 there is no need to lower the glove-box. There's an access panel in the back of the glove-box that can be popped-off giving you easy access to the air-filter cover. See owner's manual here: http://drivers.lexus.com/t3Portal/do.../sec_04-03.pdf on pdf page 39.