
A device can be marked as trusted by selecting the check box when you're prompted to enter a security code to verify your identity. Select the "I sign in frequently on this device. Don't ask me for a code." check box, and the device will become a Trusted Device upon successful login.
You can also trust as many devices as you want there is no limit.
If you don't sign in to a particular trusted device for 2 months it will be removed from your Microsoft account keeping your account secure in the event that a trusted device is lost or stolen without you realizing it. But if someone steals your device or it is lost somehow you may want to revoke that trust. This is a simple task on outlook.com - by navigating
Click your profile image in the top navigation bar
Select 'Account settings' from the menu
Open the 'Security info' category under 'Overview'.
Click 'Remove all the trusted devices associated with my account' under 'Trusted devices'
then click 'Remove all trusted devices'.

Remember, you can always trust a device again later.