I do mine for 30 minutes a day. If you are already good at meditation it won’t take long for your third eye to open and be perfected. If your new, its going to take a while just to quiet the mind.
Your chakras will open when they open. It’s not something you can rush and put a timetable on. Truth is, your chakras open and close to a degree all the time.
Here’s the deal when it comes to cosmic consciousness and the Kundalini experience. You need to have ALL your chakras open, not just the third eye. In fact, take care of your lower chakras first and the upper ones will come easy. Ignore your lower chakras and you will get nowhere, or worse, led astray.
Meditation is difficult. It can be a long journey. I would recommend beginning with 15 minutes of meditation and then increasing it to 30 minutes a day. Be sure that it is a very quiet, cool and comfortable place without distractions. Chant your mantrum and try to keep your mind and focus on the mantrum. No one can tell you how long it will take. It is best to meditate first thing in the morning before you’ve had anything to eat or have become involved with the activities of the day.
If you should have some sort of spiritual experience be grateful for it but it is good to bear in mind that experience is the exception and not the rule. If we meditate to have a spiritual experience then we are missing the entire point of meditation. It is in the stillness that that particular chakra is opened; it is not an act of the will or of the ego.
Grace, peace and love to you.
Om, hari Om. Namaste.