You must have been resident in Washington for 183 days in the tax year you are applying for in order to be eligible for the credit. This is about six months. For example, in order to apply for the Working Families Tax Credit in 2023, you need to have lived in Washington for six months in 2022. This does not need to be all at once - for instance, you could have lived here for three months, moved away, and then moved back for three months.