Do you mean at the point when they apply to YC? Around 10% currently. The number is going up, because as we get better known we get more applicants that are further along. It was 0% for the first several years.
Joking :)
Everything that can increase one's chances of getting into YC seems to be founder qualities (thats what I can make out from what has been said on this by YC in various posts). YC saying what stage a company is in impacts their selection would be a paradigm shift in how it projects itself and its selection process, imho.