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. In order to access your archived campaigns, you can go to your "Campaigns" homepage and at the top of the page you will see a Funnel drop-down box. Click on this box to switch between viewing active or archived campaigns.
In order to restore a campaign, you can click the “More” button on the campaign in question, this will open a drop-down menu. From here click “Restore Campaign.”