It depends. If amending the tax return results in a refund, then you would have 3 years from the original filing date to mail your amended return. However, if you owe during the amending process when entering the 1099-G from an overpayment of the original refund, then you would want to make sure that the amended return is mailed before April 18th, 2017 to avoid any interest and penalties associated with the overpayment.
When you mail Form 1040-X you should include the following forms:
- The new (amended) Form 1040
- Any forms or schedules that were added or changed as a result of preparing the amendment (In your specific case, you would want to add a copy of the 1099-G)
It takes the IRS up to 16 weeks to process amended returns. Also, depending on whether the changes affect your numbers, you may also be required to file an amended return with your state.
The mailing address for your state and situation can be found at https://www.irs.gov/pub/irs-pdf/i1040x.pdf. Look under Where to File on page 14.
