TurboTax will automatically save your progress as you work on your
return and your information will be stored until the end of filing
season, including extensions, so October 16th, 2017 this year.
You can go to My Account -> Save and Sign out
and come back when you are ready. However, please make sure you
remember your username as you will want to log in with the same username
you have been using (sometimes users have multiple accounts over the
years).