You can file a Form 1040X whenever you want. You don't need to wait until the next tax year. If you're to receive a refund with your corrected return, however, the IRS would like you to wait until you receive any refund you are supposed to get for your original 1040. For example, if you file on April 15 and will receive a refund, then realize on April 18 that you have to correct your return, wait until you receive your refund before you mail in your 1040X.
You have three years from your filing deadline to file Form 1040X. For example, for your 2007 return due of April 15, 2008, you have until April 15, 2011, to file corrections.
If you didn't pay all the tax you owed on April 15, 2008, then you have two years from when you paid the last of that tax liability to file a 1040X for that return. For example, you file on April 15, 2008, but you couldn't pay all your tax immediately. Let's say you pay installments for 19 months to pay off what you owe, finishing on November 15, 2009. Now you have two years, until November 15, 2011, to file a 1040X. This two-year restriction applies only if you don't pay your full tax bill by a filing deadline.
The IRS will give you the benefit of time. You have until the later of the two dates to file the 1040X. For example, let's say you didn't pay your whole tax bill on April 15, 2008, but you needed only seven months to pay off the balance, finishing on November 15, 2008. Under the two-year restriction, you would have only until November 15, 2010, to file the 1040X. But because the IRS is kind, it will give you the three-year deadline instead, because that three-year deadline will give you more time than the two-year deadline. So, you have until April 15, 2011, to file a 1040X, in this case.
If you'd like to know more about tax amendments and related topics, you can follow the links on the next page.