How long does it take for a spa to heat up?

It all depends on the season and the temperature of the water used for filling the spa. That said, if the initial temperature is 13°C (56°F), it will take, on average, around 12 hours to heat up. If the spa is connected to 120V, it can take up to 24 hours to reach a temperature of at least 38°C (100°F).