Parents receive immense joy and satisfaction from seeing their children particularly if they had not seen them for an extended period and therefore the Misva of honoring parents requires children to occasionally visit their parents. Even if a child cannot be of any assistance to the parents, and they have no interest in hearing words of Torah, he nevertheless fulfills a Misva simply by visiting them. Halacha establishes no specific guidelines regarding the frequency of such visits, but one fulfills the Misva of honoring parents each time he visits them whether he visits on a weekday, Shabbat or holiday. It is told that the Hazon Ish (Rabbi Avraham Yeshaya Karelitz, Lithuania-Israel, 1879-1954) visited his mother every day, even when she was perfectly healthy. Even a person who lives far from his parents must make a point of paying occasional visits.
A Yeshiva student who studies in the same city as his parents must periodically visit them. If, however, he is preoccupied with his Torah studies, there is room to be lenient regarding the frequency of his visits, given the unique importance of Torah learning.
Some have the admirable practice to visit their parents on Shabbat and Yom Tob, though there is no strict obligation to do so. A person who is preoccupied on Shabbat and Yom Tob with learning or teaching Torah need not visit his parents on these occasions. Even so, if they express their desire to see him he should make every effort to visit them.
If parents request that their child occasionally call them on the telephone, he should ensure to comply with their wishes.
If a person told his parent that he will come for a visit, he should add "Beli Neder," meaning, that he does not intend to make a binding commitment to this effect. According to some views, committing oneself to visiting parents falls under the category of "Nidreh Misva" making a commitment to perform a Misva and would thus require "Hatarat Nedarim" (annulment of vows) if it cannot be fulfilled. Therefore, it is proper for a person to add "Beli Neder" after stating his plans to visit a parent. If, however, the parent tells the child, "You dont have to," then the commitment is no longer binding, and "Hatarat Nedarim" is not necessary if the child changes his mind.
(These Halachot are taken from Yalkut Yosef Laws of Honoring Parents, p. 415)
Summary: It is a Misva to visit ones parents from time to time, even if one lives far away from the parents. There is no set time when one must visit his parents, but he fulfills the Misva every time he visits.
