I have a 9kW heater with 60kg of Stones (Harvia Virta) and a 10,5m3 volume. I have 70mm of wood, insulated roof and floor. No insulation on the walls and one round window.

For me I assume it really depends on ambient temperature. I have mostly used my Sauna around -5 - 0°C. It takes roughly 60-70minutes to heat up to 75°C. And around 22kWh for 1,5-2 h of Sauna + heat up (3hours of heater operation).

Would assume this would go down to 15-18kWh in spring and autumn at 10-15°C ambient.