Time Machine is an easy way to back up your Mac. If you don't perform regular Time Machine backups, however, the next one could take a while. Time Machine does its work in the background, but there is a way to see how much time remains.
From the Time Machine icon in the menu bar, you can see how many gigabytes of your data has been backed up out of the total amount. While this information is useful, it doesn't give you a sense of how much longer Time Machine will take before it's finished.
To get an estimate, open Time Machine preferences from either the Time Machine icon in System Preferences or by clicking Open Time Machine Preferences from the menu bar icon. You will need to wait for the "Preparing backup" stage of the proceedings to complete, but once Time Machine starts backing up your files, it'll show you the hours or minutes that remain.
The estimate is a moving target at the beginning of a Time Machine backup and gradually becomes more accurate as Time Machine gets further into a job.
(Via OS X Daily)