I also applied for an American immigrant visa. Last July, I found myself in the fourth stage in official website. How long do I have to wait?

When you say the fourth stage, do you mean the state of activity after the decision?

If yes, have you received the supplementary information of the interview and the notice of payment of visa processing fees issued by the American visa center?

Usually, you have to pay a fee and submit all the necessary information to the American visa center before you can start your interview waiting stage, which is estimated to be about 8- 10 months. However, if the last two steps have not been completed, it will take you 8- 10 months to complete the above two steps.