Rule of thumb is that one third of your post length should be in the ground. This can change depending on the frost line in your area.
Now your post holes are ready and you can start installing fence posts.
Your post setting is the foundation to your fence and must be done with care. No matter how nice your fence design is and how great your carpentry skill is, if the fence posts are out of line or leaning your job will not look professional.
When you are looking at the line of your fence posts each fence post should disappear behind the other. The usual recommended method for setting posts is to bring the cement to the top of the fence post hole and use crushed stone at the bottom of the hole. We find that when cement is brought to the top and creates a head, the frost has a better chance of prying it up than when it is left under ground. Example to show this point is a finishing nail and a nail with a head. Which is easier to pull up?
The gravel in the bottom is put there to drain away water from the bottom of the hole.
You will see that the way we install fence posts differ somewhat with the most recommended method. We have twenty five years of building experience using our method, building in an area that gets cold weather in the winter with lots of snow and is hot in the summer. Our wood fence posts stand the test of time, they stay plumb and don't lift.
You make your own decision on which method you will use when installing fence posts.
Digging post holes is hard work but now that each fence post hole is dug, you are ready to install fence posts.