In Minecraft water hydrates up to 4 blocks of Farmland away.
In the screenshot above, you can see how the adjacent 4 blocks of Farmland are wet and the 5th block is not.
This means that a block of Water can fully hydrate a 9×9 Farmland square (if the block of water is in the center).
The outer rim in this screenshot is made of Farmland blocks which are more than 4 blocks away from the water. Once again you can see how the outer rim is not wet, but every other Farmland block is.