It all depends when you started being self employed. If it was on or after 6 April 2017 but on or before 5 April 2018 you must notify HMRC that you're self employed by 5 October 2018. If it was on or after 6 April 2018 you have until 5 October 2019.
Having said that, there's no harm in registering sooner rather than later.
