There are two ways to answer this question.
First, Old Major in chapter one tells the animals of their lot. In light of this, the only logical conclusion is to revolt. This is also the chapter when the words of Beasts of England is given. Therefore, from this perspective, the thought of revolution is in the air. This sets the stage.
Second, in chapter two life on the farm becomes more difficult. Mr. Jones begins to drink. He loses a lawsuit and this depresses him. His men do not care for the farm, and at one point they forget to feed the animals. In addition they begin to whip the animals for no good reason. When his happens the animals cannot take the oppression any longer. Therefore, they begin the revolution. From this perspective, we can say that the revolution is only a matter of time in view of the situation on the farm.