Candidate feedback is not a concern and wasn't my question, as its always done asap - within the hour - end of day, etc. I wanted to know what is a reasonable expectation when the 2nd interview is done by another person - so HR has to get feedback from the CEO and then relay it to you. Sounds like consensus is 24 hours is acceptable - same as with a 1st interview, no matter who did the interview.
Yes, we talked about 'ideal' practices from the get go. To me, these apply whether the first or third or fourth interview. At the least, you should be directly in contact with your HR person as soon as you have spoken with the candidate - providing feedback into their decision loop is very important. Worse case is HR says ' don't know anything yet, thanks for the feedback' and you get another opportunity to set the agenda for the rest of the steps to follow (as per Jerry's comments)