I don't know, I expect it to be at least %3 as this is the general conversion rate for "free" users AFAIK.
There must be some some number that makes it viable to have free users and paid users. For games, the free users are usually those who provide the "content".
People usually demonize freemium games but IMHO its much more benign than extracting huge sums by artificially making it worse and sell attention.
