This setting only applies to Confluence 6.0 or later if you've chosen to disablecollaborative editing.
When collaborative editing is enabled we're saving all the time!
When collaborative editing is disabled, Confluence saves a draft of your page once every thirty seconds by default. Confluence administrators can configure how often drafts are saved.
As a Confluence administrator, you can set the time interval at which drafts are saved as follows:
Choose the cog icon, then choose General Configuration under Confluence Administration
Click Further Configuration in the left-hand panel.