I think it's important to remember that the Eurozone has two stages (ERM I and II) and they're both important but for different reasons.

ERM I is about achieving long term economical stability, which is established according to a series of indices called convergence criteria, that are evaluated every two years on even years. A member can technically apply for the second stage if they pass only two consecutive evaluations (4 years) with all-green marks, but the members that join nowadays do so after about a decade of green evaluations.

The goal of ERM I is to make sure a member's economy is stable to the point that switching the currency to the euro becomes moot. But it is also a win in itself (long term economical balance).

You may wonder why even bother with ERM II if every member were ERM I. Well, ERM I is what is called a common monetary policy – but ERM II is a common fiscal policy, which would mean deeper alignment and integration of all fiscal instruments across the Union.

There is no deadline that the members must meet but most EU members are legally committed to entering ERM II someday. There's no deadline because you can't wish economic stability true; each member will achieve it when it achieves it.

To give some examples:

• Denmark has been passing ERM I convergence criteria for more than a decade and can technically enter ERM II at any time and would be a formality. But they have an opt-out that says they are extempt from the legal committment to do so. Also, Danish public opinion is split fairly evenly about it.
• Croatia and Bulgaria have met the criteria for more than 10 years before they adopted the euro.
• The UK has never met the criteria between 2012-2018 so they couldn't have adopted the euro even if they wanted to. But of course they didn't want to, and also had a legal opt-out.
• Romania is one of the members that's legally committed to entering ERM II but have never managed to achieve two consecutive green evaluations since 2012. And that's despite the fact their economy is in arguably better shape than Bulgaria's (in volume and growth).