The rule of thumb is to set the posts 1/3 of the height they will support. So a six (6) foot high fence should be buried two (2) feet deep. However, central Texas has a large variety of ground densities ranging from loose with little clay to solid rock. Depending on the soil type and density a hole may range from one (1) foot in solid rock to two and one half (2 1/2) feet or more in clay or loose soil. Our goal is always setting them 1/3 of the fence height.
Because gate supporting post will have a tendency to lean towards the gate over time due to the weight of the gate pulling on it, special consideration should be given to them. A gate post should be supported by at least one other post some distance away from the side of the post where the gate attaches. It should be noted that placing two posts together or close enough that the concrete at their bases touch is not sufficient to keep the gate from leaning. When they are that close together, they act as one post.
Where it is not possible to have a second post opposite the gate side, a concrete beam can be built running under the gate to the post where the gate will latch in order to provide sufficient support.
