This answer was for the 2016 website. See the 2017 info in the next comment.
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Yes. You can come and go. Online TurboTax saves your data each time you change screens. To exit, click at the top on MY ACCOUNT, then choose "Save and Sign Out." Occasionally for some people, that Sign Out link will be temporarily frozen. In that case, then just go to a neutral screen where you weren't inputting any data and close the browser window.
When you return, log into your account and go to the 2016 Tax Timeline. Click the orange button "Take me to my return."
