I think the parties would be deciding that. They'd have to come up with their own agreement, given the circumstances each time. We wouldn't even be saying “this is the date”. When they come to an agreement, they have to agree on an end date, and they all have to agree on that end date. Then, when that date comes, they could only go past it if all parties were to agree again. Otherwise, it would basically lapse out and you would revert to the original Standing Orders.
On May 13th, 2020. See this statement in context.