Roughly what is the longest distance of spread in miles for the buddhist faith? And from where to where does this occur?

Buddhist monks made their way through silk road in first century CE and thereby Buddhism got spread across China and other parts of Asia.

Explanation:

After having its roots in China, Buddhism expanded to Central Asia, Korea and Japan. This form of Buddhism later got spread in East Asia which was known as Mahayana and then the same culture started to expand and flourish in Tibet and Nepal in the name of Vajrayana.

These countries adopted various traditions and cultures which significantly varied on the basis of Buddhist teachings and scriptures. Burma, Myanmar and SriLanka also absorbed the teachings of Buddhism and many Buddhist monks formed monasteries and began to spread the teachings of Buddha around the world.