Are you talking about T posts, wood posts, etc?  For wood posts 3-1/2 ft is about the minimum here or they start to heave with the spring thaw. I put a couple 8 ft wood posts in to about 2 ft because that was as deep as I could go with a manual auger and rock...they have since been taken out redug to 3-1/2 ft with cement in the bottom and stay where they are. 2 Ft seems to work ok for 6-1/2 ft tee posts. With either leaves 4-1/2 ft above ground which is about right.

Further south where the frost does not go that deep maybe shallower is ok.  just my experience = get them in deep enough the first time.

Jim at Dawn