how long should a backup take?

How long it takes depends on several things, including the amount of data to be backed up, the speed of the backup drive and the connection type used, USB v Thunderbolt, for example.

Using USB-A, a Time Machine backup can take a couple hours to do the initial backup of a typical HD with a few hundred GB of data. After that, each subsequent incremental backup usually only takes a few minutes.

A TM backup via Thunderbolt connected SSD drive will only take a fraction of that.

Are you experiencing a specific problem with a backup attempt?