Exactly how long it stays gone depends on the folder, Outlook recreates some folders fairly quickly.
A macro to hide folders is at the end of this article.
If you hide Junk email, RSS, or Suggested Contacts folders, you need to disable the features that use the those folders.
Before using MFCMAPI or Outlook Spy, you should make a copy of your data file!
While I don't recommend deleting the default folders, anyone who really wants to delete default folders can delete them using MFCMAPI or Outlook Spy.