An archived campaign remains archived for as long as you want it to be. You are able to reactivate it whenever you want, at any time. To access your archived campaigns, go to your 'Campaigns' homepage and at the top of the page, you will see a funnel drop-down box. Click to switch between viewing active or archived campaigns.
To restore a campaign, click the 'Actions' button on the campaign in question, this will open a drop-down menu. From here click 'Restore Campaign'.
Note: Archived campaigns still count towards your accounts campaign limit.