My iPhone 4s has gotten stuck in 'Reset and Erase All' mode for hours
If you prefer to have a flowchart to narrow down the cause of why an iOS device won't restore, try this assistant:


http://www.apple.com/support/iphone/assistant/restore/


If you prefer to have a list of steps, this article should help:


http://support.apple.com/kb/HT1808


Sadly, there are many, many reasons why restores fail, some are antivirus software being too protective, some are computer problems, some are hardware problems and others are missing drivers or just bad timing.

Once you narrow down which exact step fails in the lists above, you can ask a new question if you continue to have issues restoring your iPhone. More than 30 minutes usually means there is a problem that will prevent a good restore from finishing.