You should wait until the original return is fully processed. If you are expecting a refund and are amending for an additional refund, the IRS states you should wait until you've received the original refund. Then, you can amend as soon as the federal amended tax return form 1040-X is available for filing in TurboTax. Use this tool to check availability of forms, including 1040X: Tax form availability
Here is some more information on filing amendments: How to File an Amended Tax Return with the IRS