After you set up Time Machine, it automatically makes hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months. The oldest backups are deleted when your backup drive is full.
In the top menu click on the clock with the circle around it and the drop down will look like the image below.
If you do not see the clock with the circle around it, Go back to setup.
This is the notification you will get when the first Time Machine backup is complete.
Your first backup may take a long time, depending on how many files you have and/or if you are connected with a network cable or wireless. . You can continue using your Mac while a backup is underway. Some Mac computers make backups even when asleep. Time Machine backs up only the files that changed since the previous backup, so future backups will be faster.