Exact statistics are not available. Generally, after receiving the joint petition, the USCIS service center reviews the filing to determine whether to waive the interview - if the USCIS is satisfied that the marriage was not for the purpose of evading the immigration laws, the interview will be waived and the petition approved, otherwise, interview is scheduled. To answer your question, 100% in cases where there is a doubt about the marriage. In other cases, no one knows.
It depends on what you mean by "conditional green card petitions." If you mean the adjustment of status, the answer is 100%. If you mean removal of the conditions on the card, the answer is less than 100%.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.