The plant they are building can store 10 hours worth of electricity at maximum delivery power (200MWh @ 20MW). TFA does not say how long they can store the electricity for.
They are turning c02 from a gas into liquid form. So I assume they can store it indefinitely.
I expect it becomes financially unviable if they end up storing it for long. Daily charge/discharge allows them to amortize their annual cost of capital across 365 cycles. Weekly or seasonal storage is going to be much less cost effective.
Calm, cloudy days do happen, so we either need seasonal storage, or need to be okay with much less electricity on such days.