Brief Answer:
Essential oils are mainly plant source and are not cooking oils.

Detailed Answer:
Hi welcome to Health care magic,
Essential oils are those that are sourced from mainly plant source and are not cooking oils.
Examples are---
Eucalyptus, sandal, lavender, Ajwain, anise, fennel,carrots, Grape fruit, lemon, tangerine and numerous others. As you can see they come from common fruits and foods we eat daily. There is no need to consume pure oil.
They have many benefits--anti oxidant, flavouring, spicing, perfumes, anti cold, digestives extra.
If we are eating a balanced diet, we get most of these oils.
Some of them like lavender, sandal, eucalyptus are strictly not food items but used as perfumes and anti cold preparations.

So there is no guideline as to how long to use them. Sourced from food, we will get small amounts on a continuous basis, which is good for the body.
Regards

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