According to Twitter's inactive account policy, usernames may be disabled after they have been inactive for a period of six months. To keep your username active, Twitter recommends logging into your account regularly. Inactive accounts may become permanently removed if there has been no activity for a prolonged period of time. This means you won't be able to log in and use the account at all once it has been removed.

Keep Your Twitter Account Active

To avoid having your account disabled or removed due to inactivity, log in and post an update within six months of your last tweet. Twitter determines inactivity using a combination of information, such as your last login attempt, the date of your last post and the date the account was created. You may not be able to tell if an account is inactive without logging in, as not all account activity is viewable by the public. Twitter removes inactive accounts in bulk, so as long as you log in at least once every six months and post an update, your account should be considered active.