A pipe cleaner. A couple of times per few thousand rounds ( 3000-5000 ).
You don't need to do it very often, but it's a good idea if you shoot it A LOT in a short period of time and then plan on storing it for awhile.
The M4's used by the local SWAT teams usually have the gas tube either opened up or replaced after about 20,000 rounds or so.... And we are talking hot, heavy and full auto firing during training and the like.
It's not going to freeze up on if you haven't done it and its been 500 rounds.