Leads go "inactive" when they haven't either logged into your website or been manually updated in 60 days. You can control whether or not your inactive leads are automatically deleted by clicking Leads in the main menu, and Lead Registration Preferences in the submenu.
Make sure the toggle in the top left corner of the screen is set to Advanced, and then click the Clean Up tab.
Use the radio buttons here to choose whether your inactive leads will be made inactive after a 120 day period of inactivity, or deleted after 240 days. When you are finished, click Save Changes.