Is there a minimum time limit for the prohibition of Yichud?
Some Poskim rule there is no minimum measurement of time for the prohibition of Yichud, and therefore, it is forbidden to be in Yichud with the opposite gender, even for a mere moment, without having a Yichud breaker [i.e. Pesach Pasuach to public; Husband in city; three women; two Kosher men] available. Other Poskim however rule the prohibition of Yichud only applies if the pair will remain together for the time of “Shiur Tuma” which is discussed regarding a Sotah. Practically, if it is possible to remain in seclusion for Shiur Tuma, then even if one decides to stay in seclusion for less than Shiur Tuma, it is prohibited. [For example, it is forbidden for one to lock his office door and remain in seclusion with a woman even if he plans to open it within less time than Shiur Tuma.] If, however, it is not possible to remain in Halachic Yichud for Shiur Tuma, then it is permitted to remain together for less than Shiur Tuma.
How much is Shiur Tuma regarding a Sotah and regarding Yichud?
Shiur Tuma is the amount of time the couple need to engage in marital intercourse. The Talmud records various opinions regarding the amount of time that is defined as Shiur Tuma and the Poskim rule that it is the amount of time it takes to fry an egg and eat it. Some Poskim rule Shiur Tuma is between 5-8 minutes. Other Poskim rule Shiur Tuma is 35 seconds.
 See Minchas Yitzchok 4/94-97; Taharas Yom Tov 7; Nitei Gavriel 15/1
 Maharil Diskin; Shevet Halevi 3/182 sides with his opinion, however learns him to only be stringent in a case that it is possible to remain in seclusion for Shiur Tuma; See also Shevet Halevi 2/131
 The reason: As Chatzi Shiur is forbidden Biblically. Alternatively, due to worry that they may decide to stay for longer. [Maharil Diskin] As the Sages in the Talmud, Rambam and Shulchan Aruch did not make mention of any minimum Shiur for Yichud, and hence we can conclude that being in Yichud for even a short amount of time, less than Shiur Tuma, is forbidden. This is because Yichud is considered part of Lo Sikrav, and is hence forbidden even if it can’t lead to marital intercourse. [Shevet Halevi ibid] As the Sages suspected for hugging and kissing, which can occur with less than Shiur Tuma.
 Imrei Eish E.H. 107; Igros Moshe E.H. 4/65-16; Shevet Halevi 2/131 [unlike his later response in 3/182]; See Noda Beyehuda E.H. 160 that by a Sotah, Setira for less than Shiur Tuma is not called Setira [however see Minchas Yitzchok 4/94-97 that perhaps by Issur Yichud the Noda Beyehuda would agree to the prohibition in even less than Shiur Tuma]
 The reason: As the Sages did not suspect for hugging and kissing in Yichud, and only suspected for actual relations, which can only happen if they remain secluded for Shiur Tuma. [Igros Moshe ibid]
 Igros Moshe E.H. 4/65-16 [see there that it is forbidden due to Chashad]; Minchas Yitzchok 4/95 and Shevet Halevi 2/131; 3/182 [that so is according to Maharil Diskin, while according to the Mahram Eish even in this case it is permitted, however they conclude that we rule like Maharam Diskin]
 Shevet Halevi 2/131; 3/182 and Tzitz Eliezer 6/22-4 that so applies according to all opinions, even according to Maharil Diskin; Minchas Yitzchak 4/94-98 [Only permits this in a time of need as perhaps according to the ruling of Maharil Diskin even this is forbidden]
 See in great length Minchas Yitzchok 4/94-97
 Sotah 41
 Sotah ibid
 Michaber E.H. 178/4
 Minchas Yitzchok 4/94
 Dvar Halacha Teshuvah 11