In 2020, a decline of gross receipts of 50% or more qualifies you for the ERC. Once you've determined the first 2020 quarter where your business meets that requirement, you will continue to qualify for subsequent 2020 quarters, up until either your gross receipts return to 80% of what they were in the same 2019 quarter or the date reaches January 1, 2021.
For 2021, eligibility for each quarter based on gross receipts is calculated individually and is not dependent on eligibility in the prior quarter.