I usually take my duck eggs off the turner at day 25 and then up the humidity. More often than not they pip on day 26, or 27 at the very latest.
I have an RCOM SURO so it keeps the temperature and humidity constant at whatever I set it to. I've been told (and found myself) that you get best results at 37.2 degrees and 50% humidity days 1-25 and then up the humidity to 70% for days 25-28.