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. Occasionally, for very large stores with over 15,000 products, lots of orders and customers, etc. it can take over a day.
Subsequent backups are much faster, since we only need the information that has changed since we last backed up.
You will get an e-mail when your first store backup is complete.