Ok, for the last 4 days i've been soft resetting to find a shiny rayquaza in Sapphire. My question is this: How long does it usually take to actually find a shiny legendary?
First of all, welcome, nice nickname you got there.
As for Rayquaza, the chance of finding a shiny one is 1/8192 (or about 0.01%) every time you encounter it. That doesn't mean that after 8192 resets you will definitely find a shiny one. You may be lucky and find it on your first try, you may be unlucky and never find a single shiny Pokémon. I guess it comes down to how much time you are willing to spend looking for a shiny one.
And as George'T said, use the Q&A thread for further questions.