When is Tzeis Hakochavim after Shabbos? How much time after sunset?
In Eretz Yisrael: One is to wait a minimum of 34-35 minutes after sunset during the summer months, and a minimum of 28-29 minutes after sunset in the winter months. [Others rule one is to wait 32 minutes past sunset in the winter months and 38 minutes past sunset in the summer months. Others rule one is to wait between 45-50 minutes after sunset.]
Diaspora: The amount of time one needs to wait after sunset fluctuates according to each region. The custom in New York is to wait 50 minutes from sunset. [In general the custom is that each country calculates the leave of Shabbos based on when the sun reaches eight degrees below the horizon.] The Rebbe stated that the time of the leave of Shabbos in New York is 43 minutes after sunset, both during the summer and winter.
 The first calculation brought [not in brackets] is based on Ketzos Hashulchan 93 footnote 2; Shiureiy Tziyon of Grach Naah p. 76 and Kitzur Halachos Shabbos Miluim p. 92-100. This calculation follows the number of minutes needed to wait after sunset as ruled by the Ketzos Hashulchan, and includes the novelty of Admur that sunset begins only 4  minutes after the visible sunset which is recorded on calendars.
Biurim: Regarding the fact that at the mentioned times small stars are not yet visible in the sky, see M”B 293:6; Kitzur Hilchos Shabbos Miluim p. 100-101 for an explanation on this matter. There he explains that in truth the stars are visible under proper weather conditions, and that the main calculation is based on time, and the calculation of stars is only given when one cannot calculate the time, such as he does not know when sunset occurred.
 See Kitzur Hilchos Shabbos Miluim p. 99 that in Eretz Yisrael there are 5 minutes between the Shekia Haniris and the Shekia Haamitis. Thus we wrote between 34-35 minutes taking into account this extra minute.
 This refers to Shekia Haniris, which is the sunset written in calendars.
Shekia Haniris “the visible sunset” is 4:5 minutes before the Shekia Hamitis “the true sunset”. Hence from the true sunset one must only wait 20:21 minutes in the winter and 26:27 minutes in the summer. [Kitzur Hilchos Shabbos 293 based on Admur in Seder Hachnasas Shabbos]
 Bein Hashmashos in Eretz Yisrael varies between the summer and winter months.
 Background of above calculation:
- Sunset begins 4-5 minutes after the visible sunset recorded on calendars. [See Seder Hachnasas Shabbos of Admur, and Kitzur Halachos ibid]
- After 4-5 pass and sunset begins, Bein Hashmashos lasts between 15-26 minutes depending on the time of year. It lasts between 15-20 minutes in the winter and 20-26 minutes in the summer. [See Ketzos Hashulchan and Shiureiy Tziyon ibid]
- One is to add 4 minutes for Tosefes Shabbos.
Grand Total: Based on the above calculation in order to be sure it is always past the amount of time needed to wait one is to always wait during the winter months a minimum of 29 minutes after the recorded sunset in calendars and in the summer he is to wait 35 minutes from the recorded time in the calendar.
Ruling of Harav Hagaon Avraham Chaim Naah in Ketzos Hashulchan ibid and Shiureiy Tziyon ibid:
One is to wait a minimum of 30 minutes after sunset during the summer months [Nissan-Tishreiy], and a minimum of 24 minutes after sunset in the winter months [Tishreiy-Nissan].
VeTzaruch Iyun in Ketzos Hashulchan if this refers to the Shekia Hamitis or Shekia Haniris which is a 4-minute difference. No mention is made what Shekia he is referring to neither in Ketzos Hashulchan or in Shiureiy Tziyon. Kitzur Halachos Admur Miluim p. 104 learns in the Ketzos Hashulchan that we measure from the Shekia Haniris. Although he himself argues on this premises concluding it must refer to the Shekia Haamitis.
 Sefer Bein Hashmashos of Rav Yechial Michel Tuchensky brought in Piskeiy Teshuvos 293:4. There he also records the opinion of Grach Naah which we wrote above.
 They checked and found that three small consecutive stars cannot be seen prior to this time in summer and winter. At this time the sun is 8 degrees below the horizon.
 Piskeiy Teshuvos ibid in name of Chazon Ish and Rav Aron of Belz
 Igros Moshe 4:62; However, the Rebbe in Igros Kodesh 17:124 [printed in Shaareiy Halacha Uminhag 155 and Shulchan Menachem 2:123] after bringing the custom expressed himself there “I am not responsible for this calculation.” See there that some wanted to suggest that based on Admur one must always wait 72 minutes. However, the Rebbe negated this logic.
 See Piskeiy Teshuvos 293:4; Sefer Biur Halacha [Zilber] 293
 Heard from Rabbi Leibal Groner; See Hiskashrus 370:19; Rav Raskin in Siddur p. 230