Confluence save changes of the page into Page History whenever a user click on the Publish or Update button on the page. In this case, the frequency of the page being saved actually depends on how often the page is being Updated by the user.
If you're on Confluence 6 or above, you'll probably have Collaborative Editing enabled. This way the changes are basically saved in the Unpublished Draft everytime you stop typing for a few seconds. Meaning if you decide to close the browser or got disconnected when the changes are saved(usually take <3seconds), the changes will still be there until another user publish or delete it.
Not sure if this answers what you're looking for but do let me know if I got it wrongly :)
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