Visiting one’s parents on occasion:
It is a mitzvah for one to visit his parents on occasion and whoever increases in doing so is praised.
The frequency of the visitations: The frequency of the visitations is all dependent on the norm for one’s society in the relationship of a parent and child, and the distance apart that they live as well as the physical needs of the parents. It also depends on whether the parents made a specific request of the frequency of the visitation. Some Poskim rule that if the travel will cause one to nullify Torah study, then he is not required to make the visitation. One who is able to, such as if they live very close by, is to try to visit his parents every single day.
Who pays for the travel expenses: One must cover the expenses of traveling to his parents if he lives walking distance from them. If, however, he lives beyond walking distance, then he may ask his parents to cover his traveling expenses.
 See Admur 613:8 “One who travels to receive the face of his father or teacher… since he is going for the sake of a mitzvah”; Michaber 613:5; Yuma 77b; M”B 301:10; Divrei Malkiel 2:75; Meiah Shearim 17; Yehuda Yaaleh Y.D. 310; Hisorerus Teshuvah 4 Y.D. 9; Har Tzvi Y.D. 191; Teshuvos Vehanhagos 2:444; Nishmas Avraham Y.D. p. 150; Pesakim Uteshuvos 240:15; Encyclopedia Talmudit Erech Kibud Av Vaeim Vol. 26 p. 380 footnote 139
 Machaneh Chaim 1:85
 Meiah Shearim ibid
 Har Tzvi Y.D. 191 and Teshuvos Vehanhagos 2:444 in name of Rav Chaim of Brisk who when asked this question replied that since the son is able to walk to his parents’ home, the expenses of travel are not considered an expense on the side of his parents, but rather an expense of his own to ease his journey; Nishmas Avraham Y.D. p. 150 in name of Rav SZ”A that if he does not live walking distance from his parents then his parents may be asked to cover the traveling expenses, of the cheapest form of travel.