The Glock is so easy to detail strip, I can't imagine not cleaning it. I routinely clean parts of the Glock that I've never cleaned on my other pistols. I routinely pull out the trigger housing to wipe off and relube the trigger bar/sear interface to ensure that awesome (to me) trigger pull. And I pull the extractor and striker every now and again to clean the channels out. I wish it were that easy for all my other guns!
If you were to never clean it, I think the first thing to fail would be the striker channel getting carbon buildup, leading to light strikes. I'm not sure how long that would take, but I imagine several thousands of rounds. Depending on your load, you might eventually get enough carbon in the chamber to prevent full return to battery. There's so much leeway in the extractor grip, I don't think carbon and powder buildup could even come close to affecting extraction. And the rails are sort of self-cleaning enough I think the slide would function dry and dirty for a good long while. Maybe forever.