On average, it can take anywhere from a few minutes to a few hours to complete the first backup, depending on how many items you have in your store.
***As a point of reference, we have found that our backups - on average - can complete approximately 7,000 items per hour. For incredibly large stores, the first backup could take over a day to complete, but this doesn't occur often.***
Rest assured, all subsequent backups after this are much faster and will only take a fraction of the time - since we'll then only be saving any data that's changed since the last backup.
You will get an e-mail when your first store backup is complete. Going forward, you will then receive a Weekly Backup Summary each week that details whatever changes have been made to your data over the past 7 days.