If I have tasks and projects that I have marked completed or have not updated in a while, how long does Todoist keep them before deleting them?
Todoist stores all your data infinitely unless you go to the Preferences and completely delete your account in which case all data associated with it is removed along with all backups.
Please note, that completed tasks are stored in the "Get completed tasks" list (link at the bottom) until you click the "Delete completed" link. This will delete them permanently, but with Todoist Premium you can also access the full completion history in the "# Completed" link in the top-right corner along with a graphical visualization of your productivity.
Again - none of this data is ever removed unless you delete it yourself.
Thank you. This information is very helpful.