FAA Order 8900.2
8. Testing Applicants Trained by the Examiner. Except as noted in paragraphs 8a, 8b, and 8c below, an examiner who trains an applicant for a certificate or rating may only test that applicant if another instructor has given the applicant at least 3 hours of flight instruction and is the recommending instructor (1.5 hours if the test is in a glider or balloon; 1.0 hours if the test is in a powered parachute; 2.0 hours if the test is in all other light sport aircraft categories). An examiner may also test an applicant trained by that examiner for an additional aircraft class rating if the applicant has obtained the written recommendation of another CFI who has personally checked the applicant and found the applicant prepared for the practical test.