A traditional sauna generally takes between 30 - 45 minutes to fully heat up. During this time, the rocks will be heated to the perfect temperature for soft heat and soft steam.
An infrared sauna is not as much about the heat/room temperature, as it is of the penetration or the absorption of the infrared energy. You can technically start using the infrared sauna immediately, but if you prefer to have the room itself preheated it can be warmer in as little as 10 minutes; plan on 30 – 45 minutes to reach the upper temperature range in an IR sauna.