The Gemara in Chulin 18b says that when a person intends to return to his place he must keep the Chumros of the place he left as well as the Chumras of the place he is in.
The simple understanding of this Halacha is, that even if a person was in a place for a long time, as long as he intends to return to his original place he has the Chumras of both places. This would appear to be the opinion of the Meiri in Pesachim 51b, and is I believe, the position of most Hlachic authorities. However, some Poskim say that even if he intended to return, if he stays at his destination for a long period, he is considered Ein Da'ato Lachzor. This is the opinion of the Magen Avraham(468:12)and Aruch ha'Shulchan (OC 496:5).
A further Shitah, that of The Chacham Tzvi (167) and Shulchan Aruch ha'Rav (Tinyana 1:8), is rejected by the vast majority of the Poskim.
For further discussion, see Minchas Yitzchok (5:43), Minchas Shlomo (19) and Shevet ha'Levi (5:64).