What is the qualifying period for residence for citizenship applications?
It’s 5 years unless you are married or in a civil partnership with a British citizen in which case it is 3 years.
If you are not married or in a civil partnership with a British citizen, you also need to wait 12 months from the date of your settled status decision, if you apply using settled status, rather than an EEA Permanent Residency (PR) card.
For example, Dana is a Romanian citizen married to a Greek citizen. Dana has been in the UK from 2010 and received settled status on 2 April 2019. Dana will have to wait until 2 April 2020 before being able to apply for citizenship. If she applies in April 2020, the citizenship application will ask to evidence the last 5 years, so April 2015-April 2020. However, the Home Office may check whether Dana was lawfully resident before 2015 as well.
the3million’s Young Europeans have published a very useful document on key Citizenship facts which you can find here: http://www.t3m.org.uk/t3m_YE_KeyCitizenshipFacts.