How good does my system have to be to realize an improvement with cables?

exactly...different gear, different room, different music, room correction, different sonic preferences... all lead to different cable choice...you just gotta start trying some

Yeah, “it depends”.

"If you bought your gear at WalMart” - you can get good-n-bad gears everywhere these days.

nice and therefore expensive cables are just “nice”, not necessarily improving anything technically. I upgraded cables to make my whole system to be elegant and to manage cables to lower noise coupling, optimize loading, impedance etc.

What about the basic elements / fundamentals of music? One surely hopes for agreement / convergence.

Embrace this ⬆

Not that ⬇

My question is what's the  percentage of the overall component's cost should be the cabling and at what point should it even be considered?

Or put a different way How would you know if your cables are your weakest link?

There is no magic ratio of cable vs. component cost. Go with the flow and enjoy the journey. 

I have found the cables that made the biggest difference in my system was my amplifier power cord(s). That may not be the case in your system.