It's not a scientific answer, but I vary by show. If I'm doing a more standard park-and-bark opera, or I know the director and expect simpler blocking, less complicated cue sequences, etc, I'll do 30"'. But if I know it's a busier show with lots of stage and cue action, or if the tempo for the overall piece is faster, I'll go with 15"'. My default is probably 15"'. It's certainly more specific and pays off throughout the show in terms of paperwork, planning, etc. But it has been known to happen that my prep time is rushed, my brain is full, and I have trouble taking the time and concentration to make 15"' timings in a full 3.5-hour opera, so I just run with 30"'.