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).