Before getting into Kali sadhna, one must understand that Goddess Kali is the most fierce and intense form of the Ultimate Power, there's no one more intense than her. She is that unstoppable force who comes in life of her devotees like a storm.
Firstly, Kali upasana is not for everyone. Anyone can bow down to Kali and join hands in front of her idol, but when it comes to making a strong connection with the divine Mother, it has its own challenges.
For genuinely loving Kali, a person has to have an attitude of a Veera (a warrior sort of mindset). No matter how much hardship I have to face, I will get up and continue to fight, Kali likes that attitude.
Bhagwati Kali’s style of empowering devotees is quite different. She would liberate her devotees but not gently, she will do that with a force, that's why she has khadag and khappar with her in the iconography. She chops off the old version of a person that doesn't help him/her in life. Pleasing Kali is like climbing the mountain. Do not expect fast results during sadhna.
Yes, she brings fast changes.. But those rapid changes are for burning Karmas, Kali gives you what you need, not what you want. She first dismantles you by destroying your illusions, she is the Goddess of Time. So with time, things change, so Kali brings transformation.
Spiritual growth is fastest in Kali sadhna because the sadhna which would take decades for growth, Kali Maa can make that happen within months. The path remains tough because you will be tested physically, mentally, emotionally and you will be hit. Karmas are to be cleared for a positive transformation.
Also, since Kali is strict in nature.. She herself chooses her devotees. It cannot be a random thing to worship Kali, she cannot be called easily, if you don't have that Bhav.. no matter how much you try, you can't develop a connection with her, you need to be absolutely mad for Kali.
If the person is really crazy for Kali, he/she would feel that divine pull towards her. The devotee would have that need to look at her image or idol again and again, keep chanting her name in mind everytime or would like to talk about her everytime.
Also, when you try to develop a deep connection with Kali, people around you will be like Pishach to you, in a sense that no matter how much you love them, you might feel annoying when they would call you and you are completely in that zone, it's not like they are bad or you don't like them, but yes.. due to the fierce nature of Kali, the devotee initially might face anger issues in sadhna, but that's when you need to balance it.
Now coming to Kali mantras, if you don't have a guru, it is better not to try complex beej mantras randomly. So basically the naam mantra can help you if you don't have a guru.
This is the mantra which can chanted by anyone, it is a very powerful mantra and good for spiritual growth.
ॐ जयन्ती मङ्गला काली भद्रकाली कपालिनी।
दुर्गा क्षमा शिवा धात्री स्वाहा स्वधा नमोऽस्तुते।।
How long does it to work in a person’s life for his/her spiritual growth?
This depends entirely on the past life karmas and present life karmas of the person. Also, the individual capacity and devotion of the person towards Kali Maai.
Jai Bhavani 🙏