Saving Drafts in Outlook: Personalize Your Draft Saving Preferences

By default, messages are saved to your Drafts folder every three minutes until sent. You can adjust the time interval or location for saving unsent messages, as per your preferences.

1. On the File tab, click Options (at the lower-left corner) > Mail.
2. Under Save messages, make the following modifications:
   • To alter the draft saving location, choose Drafts, Inbox, Sent Mail, or Outbox from the Save to this folder list.
   • To adjust the frequency of draft saving, input a number from 1 to 99 in the Automatically save items that have not been sent after this many minutes box.

Source: Save a message as a file