Do Twitter Accounts Expire?by Alan Sembera
If you don't post a tweet at least once every six months, Twitter considers your account to be inactive, and may free up your username for someone else to use. However, this is not a hard and fast rule, and it doesn't mean you'll automatically lose your Twitter handle after six months of inactivity on the social media platform.
Twitter's policy is to free up inactive usernames in bulk, or in response to specific requests. So even if you don’t use your account for a long period, you still own the name until Twitter gives it away. Twitter doesn't have a schedule for releasing usernames in bulk -- as February 2014, Twitter states on its website that the first bulk release is still in progress. However, you may be able to claim an inactive username on a case-by-case basis if you can show that the handle is your brand name, company name or personal name.
After Losing Username
Even if you do lose your username, your actual Twitter account remains active indefinitely, so you can still sign in and view your past tweets. The only difference is that Twitter assigns the account a different username that's not in demand.