Aim to be fully set up at least one full quarter before your mandation date, and ideally two. The reason is not the setup work itself — it is that you want to run a live quarter while it does not count. Starting early lets you find the awkward parts of your business (mixed personal and business spending, cash takings, a bank feed that will not connect) at a point where a mistake costs nothing. It also spreads the load: HMRC authorisation codes arrive by post and can take a week or more, and if you leave everything to the month before your first deadline you will be doing your setup and your first submission at the same time. Businesses that start a quarter early almost always find their first real submission uneventful.