The truth of the matter is that repairing your credit can take some time but its surely not impossible, it takes time for a few reasons. If you discover that there are errors on your credit report or you want to challenge certain entries, the dialog via mail that you have with the credit bureaus will take at least a couple of weeks before you know the final outcome.
Especially if the credit bureau has to contact a third party creditor for information. The good thing about this is that they have a specific amount of time to do this by law. Once that time frame is up, they have to delete the item in question from your credit report. It is important that you understand the credit laws so that you know when they are in violation.
Once you begin the process, you can expect to start seeing results within a couple of weeks. You want to pull an updated version of your report often during the process to make sure all necessary corrections are being made.
Each of the credit bureaus will also send you a letter letting you know of any chances, additions or deleted items from your credit file. Also keep in mind that you have 3 credit reports from the 3 main credit bureaus. (Equifax, Transunion & Experian)
Each bureau operates independently of the other and their corresponding times will be different when dealing with each one. Keep in mind that each of your reports from the three bureaus can possibly have different information, it's a good idea to work on all three at the same time.