1. The Rambam (Hilchos Shabbos 5:20) writes that the first Teki'ah was blown at the time of Minchah. The Pnei Yehoshua (Pesachim, end of 50b) writes that this means Minchah Ketanah, which is 2 1/2 hours before sunset, because we do not find anywhere that there is a prohibition against doing Melachah on Friday afternoon from Minchah Gedolah (5 1/2 hours before sunset) onwards.

2. The Rambam does not state explicitly when the second Teki'ah was. He writes only that the third Teki'ah was close to sunset, which implies that the second Teki'ah was sometime between the first and third. We can get a more accurate idea of when it was from the fact that the Gemara states that the second Teki'ah was blown in order to instruct the storeowners to close their stores. In other words, one was not allowed to do business after the second Teki'ah. The Mishneh Berurah (OC 256:1) writes in the name of the Magen Avraham that stores should be closed about an hour before Shabbos. In the Sha'ar ha'Tzion there (#3), the Mishnah Berurah adds that this means an hour before sunset. This suggests that the second Teki'ah was an hour before sunset.

3. As we mentioned, the third Teki'ah was close to sunset. The Gemara states that the candles were lit with the third Teki'ah, so presumably this was about 20 to 40 minutes before sunset.

4. The fourth, fifth, and sixth Teki'os were blown together, and Shabbos started immediately after them, which means that these Teki'os were blown immediately before sunset.

Kol Tuv,

Dovid Bloom