In my experience the yields from poppies always seem to vary.
You can wait a few days for the highest percent chance on crops that will produce, but some will never yield pods no matter how long you wait. I know people who used to wait 9 real-life days between harvesting their poppies, but that might be overkill.
According to the Wiki it takes a
minimum of 24 real-life hours after hitting stage 5 for crops to start entering a 'secret stage 6'. So I would probably wait 2-3 extra days after hitting stage 5 before harvesting them.
