Backups take place on an ongoing basis when the computer is awake and not too busy. if you Sleep, Shutdown, or Restart your computer, or Eject the drive to take the computer to work or school, the backup simply continues where it left off when re-connected and awake.
if you disconnect the drive without properly ejecting it first, you MAY have to run Disk Utility and Repair/FirstAid the Drive directory before it can be Mounted again to proceed.
Those estimates run very high, especially the original estimate for the first Full Backup. As the backup proceeds, the estimated time remaining becomes more accurate. Your second and subsequent backups are incremental backups, and are MUCH quicker.