In On-Premises website builder installation while the customer is inside website builder editing his website, auto-backups are being saved automatically when draft is saved. It happens when the client clicks button "Save draft", "Publish" and "Download backup".
The data of all auto backup is saved in website builder installation in website project folder, e.g.: [builder]/data/e/x/example.com/backups (here we assume that domain is "example.com" and the part "/e/x/" is the 1st and 2nd letters of domain name).
By default, auto backups are saved every 5 minutes and only if changes are made to the project. The number of auto-backups per website is limited to 50 copies by default. When the limit of copies is reached, the process of saving is continued by removing oldest copies and only keeping 50 newest.

With each auto backup saving, website builder saves only data, but not the whole project, thus saving space on disk. 50 copies in total may take slightly more of disk space than the original project.

Default values — interval of 5 minutes and number of 50 copies — can be changed in On-Premises brand configuration to any values. Also there is an option to disable auto backups saving at all.