In the siddur there is a beracha to say upon seeing an exceptionally beautiful person.
How beautiful must said person be?
May I say it on either gender?
Do I say this in front of the person or do I say it discretely?
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The Talmud (Brachot 58b) says that upon seeing good creations or trees one says ברוך...שככה לו בעולמו.
The Tur (OC 218) quotes an argument between the Raavad and Rosh if blessings like these should be recited only the first time one sees the object, or even if the object hasn't been seen in 30 days. He rules like the latter opinion (Rosh).
The Tur in OC 225 records the OP's blessing and (just prior to it) its opposite: the blessing on strange looking creations (משנה הבריות). In discussion each the Tur mentions that the Raavad thinks they are only said once, and by משנה הבריות (which he listed first) he adds that as discussed in OC 218 we don't rule like the Raavad. No mention of the Rosh's opinion is made by the OP's blessing. The Shulchan Arukh uses an equally imbalanced formulation.
Some (eg. Bach) derive from this that only by the OP's blessing does the Tur rule like the Raavad. Others (eg. Gra) find it odd that the Tur would rule like different sides of the same general argument in different places. The Mishna Berura rules like the Gra, that the OP's blessing and its opposite משנה הבריות have the same rule.
(In terms of משנה הבריות, the Mechaber and Elya Rabba seem to rule to only say it once, while the Rama and Gra maintain it is said every 30 days. The Mishna Berura and Arukh haShulchan seem to leave it in doubt. CYLOR as modern opinions vary.)
The Elya Rabba (and the Arukh haShulchan after him) note that this blessing can be said by a male even on an idolatrous non-Jewish woman as evidenced by the Bavli (Avoda Zara 20a) even though a man shouldn't be staring at woman and should not be praising idolatry.
As for how beautiful, Rav Ovadia Yosef apparently thought a nice parakeet should do the trick. Rav Ovadia of Bartenura (Avot 3:7) quotes an opinion that the type of nice tree may be encountered on a seemingly regular walk. This kind of thing is very hard to quantify. If you feel a need to ask about saying a blessing, you are probably in the right ballpark.
Rabbi Lazer Brody says as follows.
This is a type of beauty that’s so special that your heart skips a beat or your soul feels totally enthralled.
You only make the Bracha the first time you see the person, and then you don't make it anymore again on the same person ever.
A man should not make this Bracha on a lady because he shouldn't be looking at women.