If it discharges continually, it will discharge for 80 hours. 8000kWh / 100kW, capacity divided by discharge rate

This is a different number than the amount of energy lost when it is not discharging. However I would suspect that this particular unit is not designed to store heat for six months.

However there are plenty of underground systems designed for seasonal storage, and the storage capacity is relatively cheap. The problem is getting enough neighbors together to meet the minimum scale.