Answer: You will have dug your hole at least 3 FT deep. How much cement you use depends on the amount of dirt you have taken from the hole. If you dig it with a shovel you will probably need more cement, probably 4 bags. If you use a post digger or motorized auger you will probably need around 3 bags. If you are using either a cylinder or free pouring be sure to bring the cement up to either ground level or very close to it.
Free pouring will use more cement but it will hold the post in place better. If you use a cylinder after they are filled with cement you have to fill up the surrounding area outside the cylinder with dirt. It is difficult to compact the areas so after the cement has hardened you may see that you can move the post. That is because the soil surrounding the cylinder may not have been compacted enough.