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. 

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.