Pure essential oils are 100% strength, they are extremely potent hence should not be applied to the skin unless diluted first.
Effects can be immediate.
If using the same blend of oils for a particular reason you should take a break from it after a few weeks, before returning to it so that you don't sensitise your body to the oils.
I will give you an example of how strong these oils are.
I have normal blood pressure, sometimes its slightly low but generally normal.
If I burn grapefruit oil in an oil burner, I get immediate heart palpitations, so imagine someone with high blood pressure getting that reaction?!