there is no deadline for interviewers to get their forms back. but the adcom is good about getting people done in a reasonable amount of time... sometimes even if they have to bug the interviewers, but i'd guess that some interviewers just don't get back as fast.
JDAD... if it has been 8 weeks... i'd give them a call. be nice! just mention that you interviewed 8 weeks ago and wonder if you'd expect an decision back soon. good luck.