Standing up for a parent:
When: Some Poskim rule that one is obligated to stand for a parent when he hears the parent enter, even if the parent cannot yet be seen. Other Poskim however rule one is obligated to stand for the parent when he/she enters one’s sight. The above applies even if the parent is much further than four Amos distance.
How often: Some Poskim suggest that perhaps one is only required to stand for a parent twice a day, one time in the morning and a second time in the evening. Likewise, in the event that one lives in the same home as his parents, he is only obligated to stand when they come home from work and the like, and not every time the parent is seen in the house.
Forgives his honor: If a father [or mother] forgive their honor, their honor is forgiven, [and one is not required to stand on their behalf]. [Seemingly, based on this many children are lenient not to stand for their parents, under their assumption that their parents forgive their honor. Nevertheless, it is best to receive explicit permission from one’s parent. As stated above, the Rebbe was accustomed to stand for his mother whenever his mother would enter the room.]
 Michaber Y.D. 240:7; Rambam Mamarim 3:6; Kiddushin 31b and 33b; See Kibud Umoreh chapter 9
 Pischeiy Teshuvah 240:7; Aruch Hashulchan 240:24
 The reason: This is included in the Mitzvah of honoring one’s parents. [Rambam Mamarim 3; Aruch Hashulchan 240:24] Alternatively, it is included in the Mitzvah of fearing one’s parents. [Pischeiy Teshuvah 244:3]
 Heard from Rabbi Leibal Groner; Rav Ashel Lemel Hakohen told me that he too was accustomed to stand for his father/mother when they would enter their home.
 Kiddushin ibid; Gilyon Maharsha 240 based on Rama 282:2; Aruch Hashulchan ibid
 Shevet Halevi 2:111-4, based on Shach 282:3
 The reason: This is similar to the ruling regarding standing for one’s main teacher in which we follow Malei Eiynav.
 Chayeh Adam 67:7; Aruch Hashulchan 240:24; Shevet Halevi 2:111-4
 The reason: As the honor of his parent should not be any greater than the honor of his teacher or Hashem in Kerias Shema. [Aruch Hashulchan ibid; See Taz 242:12]
 Aruch Hashulchan ibid
 Michaber 240:19; Kiddushin 32a
 See Kibud Umoreh 9:6