They’ve got a quote from an Oxford uni professor of energy and climate policy in there who says it can store heat effectively for weeks. The manufacturer claims that it maintains about 80% of its “charge” if left for a month

One of the cool things about this design is that the bigger it is, the better it gets at storing heat. The square-cube law is in effect here, since heat can only be lost by leaving the silo at the outside whereas heat can be stored throughout the whole volume