Source: al-Fatawa al Iraqia Ibn Taymia vol. 1, p. 141
A traveler arrives at its destination, and know that he will stay there 10 nights or more. Is it permissible for him to shorten and combine prayers?
What is Sunnah for a traveler is that he shortens the prayer to two rak'a except Maghrib, and combine prayers when needed.
If the traveler arrived at its destination, it is Sunnah to shorten the prayers, but not to merge them unless there is a need for it.
And if he does not know how long he will stay as he continues to shorten the prayer (indefinite) but if he knows he'll stay five, ten or fifteen days so they have learned two different opinions and it seems to be most accurate is to He also then shorten the prayer, and Allah knows best.