With essential oil and How long essential oil lasts can be like asking how long is a piece of string. There are many variable factors that contribute to how long it lasts. Firstly each bottle contains 10ml of essential oil and how often it is used and how strong you like the aroma of your steam. For the purposes of this answer, we will take an average.
Approximately 5ml of essence will give you around 10-15 minutes of fragrant steam. There will remain a hint of the essence for a period of time after. With a hint already present it will require cleansing should you be changing essential oil blend. Of course, the amount added should be adjusted based on the amount of time you will spend steaming and of course for how strong you like the fragrance.
Top Tip: Continue using the same essential oil across different steam sessions to prolong the aromatic experience and avoid having to continuously use the cleanser, changing essential oil.