The amount of time that you will remain logged in is called a session. How sign in sessions are handled depend on whether you're signed in on the app or the desk.
Desk sign in sessions
Users signed in on the desk view will remain signed in unless the following conditions are met:
7 days of inactivity have passed, where inactivity is determined by whether they've accessed and viewed any parts of their workspace within that time period.
The user account accessing the workspace has been made inactive
The email address or password on the user account was changed
App sign in sessions
When a user signs in on the app, instead of session time an authentication token for the device is generated and will be held until you've signed out of the app manually. While signed in, if any changes are made to the workspace name or the user account then the app will display error messages explaining why data couldn't be refreshed. Provided the user account is still active, this can be resolved by signing out of the app and signing back in.
Can I be signed in to multiple devices at once?
Yes, however this is not recommended as this can create conflicts when making changes to tasks, properties, etc. leading to errors or the wrong information being synced back to the the workspace. For this reason, it is highly recommended to only be signed in and operating on one device at a time.