WPMU has a built in autosave feature that automatically saves when the author is adding/editing a post. By default, the autosave kicks in every minute and on my computer/connection averages about 3 seconds to complete. You can alter the autosave behaviour by adding a couple defines to your wp-config.php:
The autosave interval is in seconds. An interval of 600 is 10 minutes. I would not have the autosave interval set to less than 5 minutes. 10 minutes is still a fairly reasonable interval.
A second feature that has been added is post revisions. A user can access these via the bottom of the write post area. If post revisions go unmanaged, they are likely to significantly enlarge the size of database.
With this define WPMU to only keep 3 revisions. When the 4th revision is saved the first is purged off. Hence, it will not enlarge database as it was. You can change the wp_post_revisions settings upon the requirement.