After a 30 min fast walk how long does or should it take to bring your heart to a resting rate. How many mins?

As a general rule... ...once you feel rested & recovered from exercise, the heart rate, respiratory rate and blood pressure should return to pre-exercise values. For a healthy person after moderately intense exercise that increases the heart rate by 50%, 10 minutes of rest should suffice. But I must emphasize, this is a GENERAL rule. Hope this helps some.
It varies. It would vary based on the individual's level of physical fitness or any other medical conditions. I would expect within 15 minutes or so of cooling down. If the heart rate persists at above 100 for over an hour, I would see your doctor. If it stays high and you have symptoms of dizziness, nausea, or passing out, go to the ER immediately.