How to know whether I've attained mantra siddhi?
How do I understand that how far I'm in attaining it?
Its important to understand what "Siddhi" means. Siddhi which means accomplishment may refer to different things based on context.
When a mantra sadhana is done for fulfillment of specific sankalpa/vow taken, then accomplishment of that sankalpa is considered the siddhi of the mantra.
When a mantra sadhana is done with a desire to attain the shakti/power associated with that mantra/deity, then accomplishment of such power is considered as Siddhi.
But the real siddhi is when, the mantra which is practiced will lead to the realization of the Deity, and when one finally merges with the deity. God realization is the ultimate siddhi of any mantra.
Well, there are no definite steps or levels to evaluate, if you are making spiritual progress, you will notice that, you will become more detached, less anger and you will feel contentment. If these are happening, then there is a definite progress.
As the Japa makes progress, ultimately a stage will come when japa will become ajapa. That is, you will practice repetition of mantra spontaneously without any voluntary efforts. Then, the mantra will be playing in the background 24*7. That can be considered as a major step in the spiritual progress.
My experience, I dont know what stage i am in but i do see certain changes.In the beginning uttering mantra was merely like recital of words.gradually there established an immense connect with the deity. poetry over flows some times looking at the incredible beauty of the deity.Sometimes i go in trance .sometimes the mantra chanting stops after one round itself and takes me to absolute stillness and makes me want to see him alone.At that point i see him alone and no other God's who are around.