While writing a post in a wordpress admin side, wordpress will automatically save your posts as a revision using AJAX. By default this interval is set to 60 seconds.
You can modify this time using wp-config.php.
You can increase this interval for lower numbers of revisions, or you can decrease this duration to make sure that you will not loose any of the content.
Place below line to your wp-config.php file.
Note: It is good to save revisions, but interval should be reasonlable otherwise it will be overhead for your database.