Genesis 16:3

And Sara the wife of Abram took Hagar the Mizreitha handmaid, when Abram had dwelt ten years in the land of Kenaan, and set her free, and gave her to Abram her husband to wife.

AT THE END OF TEN YEARS. This is the established period for a woman who has lived with her husband for ten years without having given birth to children, after which he is bound to take another.

AFTER ABRAM DWELLED IN THE LAND OF CANAAN. This tells us that the period he dwelled outside of the Land is not to be included in the count of those ten years since he was not told And I will make of thee a great nation until after he had come to the land of Israel. Thus the language of Rashi.
This reason is not proper since it is a clear-cut halachic decision that the time spent living outside the Land is not to be included in the ten year period for any person in the world., was made to Abraham after he had come to the Land of Israel?” This language would make it appear that only in Abraham’s case was the time spent living outside the Land excluded from the ten year period, when it really applies to everyone. The Mishnah containing this principle applies to all men. And if it were as Rashi stated it, i.e., on account of this promise made to Abram, then for other people [the years they dwelled outside the land of Israel] should be included in the ten years period! Some Talmudic commentators have already made another mistake concerning this rule, stating that the law does not require a person who dwells outside the Land to divorce a woman with whom he has lived for ten years without her giving birth, nor does the law require him to marry another woman. But the matter is not so. Rather, the intent [of the law which excludes the period one dwells outside the land of Israel from the ten year total] is that if a man lived with his wife for five or ten years outside the Land and then they came to the land of Israel, we give them a period of ten years from the time they came to the Land, for perhaps due to the merit of the Land they will build up a family. And thus did Abraham and Sarah our mother do from the time they came there.

ותקח ... מקץ עשר שנים, ten years had elapsed since Avram and Sarai had moved to the land of Canaan. She had waited all these years thinking that G'd would give Avram a son from her in that land. After all, the instruction to Avram to go forth from his birthplace, etc. had been followed by the promise that there G'd would make him into a great nation. (Genesis 12,2). Our sages in Yevamot 64 state that if someone was married to a woman for 10 years during which she was unable to bear a child for him he is not allowed to simply forego the chance of having children, but should pay her out her marriage settlement (Ketuvah) and divorce her as it is possible that G'd only decreed that he should not have children from that particular woman. The Talmud, while admitting that there is no proof for the above, claims that Avram's example can serve as a guide for such conduct, seeing that he waited ten years after settling in the land of Israel, having discounted the earlier years during which Sarai had not conceived.