What is the ruling of Islam on a person and his family being away from his parents for many years in pursuit of livelihood outside his country, and does Islam specify a certain period of absence after which the person must return?

There is no specific duration for being away from home. The fundamental principle is that an adult is master of his own affairs and may travel to seek lawful sustenance without the permission of his parents, unless this causes harm. Some scholars are of the view that traveling for trade without the parents' permission is permissible if it brings the traveler more than he would earn by staying. However, if the travel is for the purpose of accumulating wealth while he is in a state of sufficiency, it is not permissible even if they grant him permission. If the travel is to fulfill his own needs or the needs of his family, he may disobey them.