Ukraine

It all depends on how close the country already is to meeting the requirements for EU membership. The EU basically just started the process of assessing Ukraine and Moldova on these. If a candidate meets some requirement, great, otherwise, both sides discuss which reforms might be necessary. If I had to make a guess, I would expect this process to take decades, at least for Ukraine.

Additionally, accession still requires approval by all member states. Even if Ukraine were to meet all requirements, any existing member could block it from joining.