How Often Should I Clean My Filter Tubing

You will find that after a while, when you turn the filter off and back on again, a lot of gunk is released into the tank. This is build up in the hoses being dislodged and indicates when a clean is required 

Transparent tubing sounds really useful. 

Tube cleaning may also depend on your tank set up.
I clean the tubing on my river tank every 3 months(ish), but that tank has a lot of algae eaters that create massive amounts of pooh, and the tank has strong water flow, so a lot of muck ends up in the filter. I will also admit that the filter makes tube cleaning easy as it's very straightforward to swap the out hose to the intake port when cleaning, so any muck ends up in the filter rather than the tank. I will also admit that I may not clean the tubing as regularly in it was more difficult. 
My axolotl tank has a smaller version of the same filter as my river tank, but I probably only clean the tubing on that one perhaps once a year. This is because the axolotls do massive individual poohs that sit on the sand, and get removed with a turkey baster thingy. The plants in that tank are large anubias, so any plant waste is usually involves a large leaf being removed from the surface.
Again, not really answering your question, but as JPC says, I don't think there's a simple answer. I will add that it may be better to start by cleaning the tubing perhaps quarterly (if the tubing is not transparent), and make sure that the mess doesn't go back into the tank. You can then monitor the situation and decide what tube cleaning schedule suits your set up.