Ann: ADH Trading Update and Stores Re-opening, page-73

Here's the ATO version:

You only need to satisfy this requirement once – you don't need to retest your turnover each month.
However, you will be asked each month to tell us your current and projected turnover.

At the time you enrol in the JobKeeper Payment scheme, you need to confirm that your business in a relevant period has had, or is likely to have, a:
30% fall in turnover (for an aggregated turnover of $1 billion or less)
50% fall in turnover (for an aggregated turnover of more than $1 billion), or
15% fall in turnover (for ACNC-registered charities other than universities and schools).

Given they qualified in March 19 vs March 20, or April 19 vs April 20, they get it for 6 months.