No, no, no. It does NOT take "3-9 months after receiving supporting documents", as the other poster claims.
Per the Fair Credit Reporting Act (FCRA, filed under 15 USC 1681), when you file a dispute with the credit bureaus claiming that an item in your credit report is false or inaccurate, they have 30 days to investigate, starting from the day they receive your dispute, and must provide notice to whoever reported the information with the first 5 of that 30 days. Filing it online just starts the clock quicker. If the 'supporting docs' show that the item is "inaccurate, incomplete or cannot be verified" the credit bureau must either delete the item or modify based on the results of the investigation. Regardless of which action they take, they are required to promptly notify you of the
They do get a 15 day extension if, during that 30 day period, you provide them with new info relevant to the investigation. However, if the company that reported the bad info doesn't respond or drags their feet and the 30 days passes, the credit agency is required, by law, to remove the item.
Be aware that the credit agencies don't always delete the info like they should. If they don't delete the entry or change it properly, they are in violation of the FCRA. If that happens, write to them and tell them that their failure to remove info shown to be inaccurate, or failure to notify you of their investigation, is a violation of the FCRA and will subject them to legal action if they don't comply. Usually, that's all it takes.