The time needed for a Time Machine backup depends on the amount of the data on your Mac, the size of the backup, the connection speed between your Mac and the Time Machine backup disk, and if it is an initial backup. I made an initial Time Machine backup of 38 GB with my 64 GB Toshiba USB 3.0 flash drive in macOS Big Sur. Preparing backup only takes one or two minutes, and the whole backup finishes in two hours.