It looks like there is a 'requirement'Is there such a requirement?
Hungary, Croatia, Czech Republic, Poland, Denmark, Sweden aren't part of the eurozone
https://ec.europa.eu/info/business-economy-euro/euro-area/enlargement-euro-area_en
but, in practice, you can take as long as you like over itAll EU Member States , except Denmark and the United Kingdom, are required to adopt the euro and join the euro area. To do this they must meet certain conditions known as 'convergence criteria'.
For exampleThe Treaty does not specify a particular timetable for joining the euro area, but leaves it to Member States to develop their own strategies for meeting the condition for euro adoption.
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52014SC0341&from=EN
And I don't see any reason why the UK's existing opt-out wouldn't still be available - but trying to look that up is too much effort!Sweden does not have a target date for adopting the euro. The preparations were stalled by the negative outcome of the referendum of 14 September 2003.