I think the amount of time varies from diocese to diocese. The diocese I live in requires a year waiting period after the divorce before you can file for an annulment or a separation of 18 months prior to the divorce. My priest says that you send in a preliminary set of questions. These are sent in and reviewed for about 3 months. Then they let you know whether they think you might have grounds to believe that your marriage was invalid. They send you another packet of questions that is much longer. They review those papers. They interview your witnesses. They talk to you. Then they make a decision. All of that takes about 6 months. Then they send it to another diocese who makes the final decision. The final decision takes about a month, but our diocese is small and the other diocese is small, as well.
Be patient.

Use this time to learn about what went wrong in the first marriage. Then study about God’s purpose for marriage and the habits of successful marriages.
Remain strong and God Bless.