What amount of actions should a user perform to earn privileges

My question is related to "virtual economy system" so it's kinda political.

I have a question and answer website like SE, however, there are both free and paid questions. An OP can devote money to post a paid question, and the user who answers it, will earn that money.

Users earn the privilege to answer paid questions by answering free questions. How many free questions should they be required to answer? Here is my suggestion:

  • 3 answers per day
  • 15 answers per week
  • 20 answer per month
  • 50 answer per year

Just to be clear, "3 answers per day" means that a user must answer 3 free questions each day to be able to answer non-free questions.

Note: I just count the answers that have either zero votes or a positive vote score plus answers that are written in the last hour or before.

Well are these numbers fine? Or should I increase/reduce them?