eh it depends... if u have a lot of gunk... i would do it about every month, bu if its generally clean once every 2 months take apart everything and clean it all out! runs quieter and better plus it wont smell!
If you have lots of gunk, that means you have a tank issue. Instead of curing the symptons of the problem such as cleaning the tubes, you should cure the root such as under filteration, over feeding, excessive algea...etc..etc.