The table below shows average heat-up times for various outputs of heaters for pools and spas.
Important: Remember that, unlike gas heaters, heat pump outputs are based on an ambient temperature of 27-28°C. Heat pump output drops as the ambient air temperature drops. For example, on a 15°C day, a Heat Pump will drop off approximately 30% (1/3), so a 10kW Heat Pump can only product ~7kW of heating.
Gas heaters have a fairly consistent output regardless of temperature.