You have to set the interval yourself, just pull down the menu bar at the top and hit the auto save button. You can tell it to keep a series of saves, 10, for example, that are saved at intervals of 5 minutes or whatever you choose.
I think if you save manually, the older saves are thrown out, unless you tell it not to.
I learned to use it after ruining my work then working on oblivious for several days, until the crucial moment I found out that I was a halfwit...
Also, I'm not 100% sure, but I think one of the latest versions of Blender had some trouble exporting correctly to DBP, so I'm using normally one version behind the latest - anyone else found this to be the case?
I wish I knew what I were doing!