As a mystic I’d say that philosophy is a great place to start, the unexamined life is not worth living. In order to be a mystic all you need to do is simply turn over your metaphysical ideas to your subconscious and wait for moments of epiphany to enlighten you. It’s a wonderful process.
So the answer to your question is ‘as much as possible,’ and ‘as little as you need.’
To understand mysticism you simply need to be introspective, study your own mind, study the nature of your being, your body and your environment with a view to understanding how all of these things are not only interconnected but are manifestations of one thing, that one thing being everything that can possibly exist.
You are a manifestation of the universe and thus are the universe in miniature in the same way that a tree is an entire forest wherever the tree happens to be or a wave is the entire ocean wherever the wave happens to be, thus it seems that you are the entire universe in the place that you call ‘here and now,’
Philosophy = How to live a good life
Religion = How to correctly prepare for the afterlife.
Mysticism = How to become divine.
Philosophy and religion make good use of the rational faculties that human beings possess to reason out their place within the universe as well as the meanings and purposes of living a good life or assuring a place in the afterlife.
Mysticism on the other hand turns us away from the rational faculty to a more subconscious and by extension intuitive understanding of mankind’s relationship to the cosmos and his essential divinity. Most mystical revelations bubble up through the subconscious as moments of epiphany that suddenly cement a new understanding of your true nature into your conscious mind, other forms involve dreams and visions.
Mysticism is the process of uncovering your true self in the same way that an archaeologist might brush away at on odd shape in the soil to reveal a beautiful statue or other prized artefact that has been entirely hidden and now revealed to be perfect.
As a caveat though there’s more than one way to climb the mountain, don’t make the mistake of thinking that your revelations are true for everyone, they are probably not, they are simply true for you and represent the best path to unleash your inner divinity. Every flower grows in a unique way and yet they all point at the sun.