If you want Time Machine to automatically make future backups, check the box on the left side of the settings pane labeled Back Up Automatically.

If your backup drive is already configured for Time Machine, this box may be checked automatically after you select the disk.

If you select Back Up Automatically, Time Machine will make backups once an hour, and keep backups as follows:

Hourly for 24 hours

Daily for the past month

Weekly for all previous months