From when until when are you allowed to make the bracha,and what is optimum?
The bracha can be made from the time the tree starts to bloom, regardless of the month (Mishna Berura quoting "Acharonim"). Lechatechita it should be said before the fruit grows.
The Mishna Berurah sides that, bediavad, if you did not see the blossoming and the fruit started to grow, you can rely on those who say you can make a bracha until the fruit is fully complete (minimally edible?). It sounds like if you saw the blossom and passed up the opportunity, you should not rely on this opinion. The Aruch haShulchan does not side with that qualification.
A lot more sources posted at Dose of Halacha
According to some poskim (Halachos Ketanos 2:28; Kaf Hachaim OC 226:1) the beracha should only be recited in the month of Nissan, though most poskim (Ritva, Rosh Hashana 11a; Mishna Berura 226:1; Aruch Hashulchan OC 226:1; Tzitz Eliezer 12:20:3) write that one should say it whenever when one first sees the blossom. R’ Ephraim Greenblatt (Rivevos Ephraim 5:173) writes that he witnessed R’ Moshe Feinstein say the beracha in Iyar.
Thus, R’ Ovadia Yosef (Yechave Daas 1:1; Yabia Omer OC 8:8:4) writes that one should ideally wait until Nissan to recite the beracha, though one can still do so afterwards. R’ Yitzchok Yaakov Weiss (Minchas Yitzchak 10:16) writes that where trees blossom at different times, such as in Australia, one recites the beracha then (See Har Tzvi OC 1:118; Teshuvos Vehanhagos 1:190).
While the Kaf Hachaim (OC 226:9) writes that one can only recite the beracha upon first seeing the blossom, most poskim (Machatzis Hashekel 226; Shaar Hatziun 226:3; Halichos Shlomo 1:2:5) write that one can recite it later if necessary.