Sheikh Salman al-Oadah
There is considerable scholarly disagreement on this matter. Scholars have offered a number of different opinions about the maximum period of residence before a person ceases being considered a traveler and must perform his prayers in their full form. opinions vary from three days to four, to seven, to ten, to fifteen, to seventeen, to nineteen, and to twenty days.
Other scholars, however, have said that the period residence for a traveler cannot be determined in days. What must be taken into consideration are the circumstances that the person is in.
Those who set a certain maximum period for travel - the majority of scholars, who nevertheless disagree on the number of days - have in their favor a precise and tidy ruling.
Those who do not set a fixed period have the evidence in their favor, since there is no clear-cut hadîth evidence establishing a maximum time limit. However, a problem arises because different people have different ideas of how travel and residency should customarily be defined. Some have taken the idea so far that they considered all sorts of people as travelers - students, ambassadors, and even some workers staying abroad for months and years on end. This is undoubtedly a problem.
I support the opinion that prevailing custom should be used to define who is a traveler and who is a resident. Whenever a term is used in Islamic Law without being defined, then its precise definition is to be determined by custom.
Therefore, if a person is moving about and not staying in one place for a long period of time, then he is a traveler and he will shorten his prayers as long as he is in such a state, unless he prays in congregation with others. He is also permitted to break his fast in Ramadan.
People, like students, who stay in one place for months or years on end have to consider themselves as residents, since they are virtually like the residents of the place where they are staying. They rent apartments on yearly leases and do everything else that a resident does. Students and ambassadors are not people who move frequently and have the intention of returning to their homes or going somewhere else at any time.
It is worth saying that this is a matter of scholarly disagreement, so no one should find fault with what one's fellow Muslims choose to follow in this matter.
Source: Islam Today