The ones I've hatched in bators always go 28 days like clockwork - lockdown at 25 days. Very high % hatches.
These things are hardy! I've had clutches hatch in the pouring ration in tall grass and they are amazingly resilient. I no longer use a bator for guineas because they hatch so many on their own. I give away hundreds every year. But give them away young because they will come back if you don't.
Ok when you start giving them away send some my way!! I have horrible success trying to hatch anything here!!