Your gas tube never really needs cleaning unless you've spent time in a swamp with your gun underwater, and even then I dont think it would need cleaning.
You see, every time you fire a round, so much gas is fired through the gas tube that it almost cleans it out. In other words, any clogs or carbon that's starting to build up, gets blown out the tube.
That being said, for the last year I've been taking a long, white pipe cleaner and checking how dirty my gas tube from the Upper receiver (i.e., without removing the gas tube) once every 5,000-10,000 rounds. ---This has kinda become an experiment of mine.
What I have found is that each time, the pipe cleaner gets a little dirty (just barely) but is still fairly clean. I've done this several times now with several guns.
Frankly, I've seen AR-15 Uppers that have 20,000+ rounds through them that have never had the gas tube cleaned. And they still seem to work just fine.
My suggestion: If you really want to see how dirty your gas tube gets, wait until you have 5,000 or 10,000 rounds down the pipe and then check it, but clean it from the upper and dont remove the gas tube. Otherwise, dont clean it... It's probably the last part on your gun you have to worry about cleaning.