The Maximum Duration slider sets the maximum number of minutes that can elapse between Auto Saves. If you do not save the project yourself within this time, ZBrush will then do so automatically.
Any save, either manual or automatic, will reset the time. This keeps you from encountering a situation where you save your work only to have to wait shortly after that while ZBrush performs an Auto Save.
Auto Save can effectively be disabled by setting this and the Rest Duration sliders to their maximums and storing the configuration.
The Rest Duration slider sets the maximum number of minutes that ZBrush can be idle before it will execute an Auto Save. If you set this slider to 1, then ZBrush will watch for one minute of inactivity. If you then leave your computer, or switch to another application, for that length of time, ZBrush will execute an Auto Save.
The Rest Duration is totally independent of the Maximum Duration setting. If you save your work immediately before stepping away from your computer, ZBrush will still do an Auto Save after the specified Rest Duration.
Auto Save can effectively be disabled by setting this and the Maximum Duration sliders to their maximums and storing the configuration.
With the Skip History option enabled ZBrush will not save undo history with either Auto Save or QuickSave files.
Max QuickSave Files
The Max QuickSave Files slider allows you to set the number of saved iterations you would like ZBrush to QuickSave. By default this is 10 but can be adjusted as high as 100. You will be able to access these saved files in LightBox under the QuickSave tab.
These files are located in the public ZBrush folder. On Windows this is the C:Users\Public Documents\ZBrushData\AutoSave folder. On Mac OSX it is the Users/Public/ZBrushData/AutoSave folder.
Delete QuickSave files
When the Delete QuickSave files button is pressed all QuickSave and AutoSave files will be deleted from your computer. Note that the files are deleted completely, not sent to the Recycle Bin/Trash.