you dinks are all wrong.  it's not about how good you are, it's about who you know.
but more seriously, when you are ready you know it.  when you watch ski videos and you know that you are better then 70% of the talent, you are ready.  it's not just about talent, it's also about marketability and recognition.  current pros are already established names, for you to get sponsored means that you have to be able to convince a company that you are a better investment then their current riders.  you don't have to be good enough, you must to be better.
you also have to prove that you will gain exposure for the company.  so what if you're sick, if nobody know who you are, how are you going to help the company?  ski companies are mostly euro controlled rich bitch nit pickers who don't give a fuck about you or freeskiing.  you have to be able to improve their bottom line.  this means you have to have exposure in videos, magazines, and competitions.
quite frankly, when it comes right down to it,  by the time you're ready to get sponsored, they'll probably be already knocking on your door.