How do I know my elements are balanced? How long to invoke an element? One week?

You can look at the elemental distribution in your natal chart. Most everyone has some sort of elemental imbalance which can be corrected with invocation of the missing element, or just invocation of Aether and programming it to balance all the elements in your soul.

Elements can take time to balance, like described on the JOS website. You can invoke the element every other day for a week or 2 and then balance it out with the opposite element if need be. Start slow (4 - 7 breaths) and see how much the element invocation affects your character and mindset. This also takes a considerable amount of knowing yourself and who you are.

Be weary about the fact that sometimes the effect of the Element will wear off and you'll need to invoke it again at a later date.

Take your time, know thyself.