Until would mean a different thing. The verb should be changed to a different conjugation for it to make sense:
- Até as meninas falarem: until the girls talk. (corrected later: that's personal infinitive)
Até que as meninas falem: until the girls talk (present subjunctive)
Até as meninas falam: even the girls talk. (simple present)
That's why "until" cannot be accepted as an answer here.
I translate até as "up to" and then it makes since. It can mean up to a time (até amanha) our up to a limit (até as meninas falam, Eu vendo até meu relógio). I'm assuming but I'm not sure, that it can mean up to a quantity (cheia o copo até aqui) It sounds funny in English, though, because we typically use even and until where apparently Portuguese uses different tenses to make those distinctions.
That's because, as someone above said, it's not a preposition. It's an adverb. The closest way to describe it is in a cousin language, Spanish: "hasta", which is also used in some contexts as "until".
The general idea is that when you use this word as an adverb, it puts more emphasis on the action, giving it a surprised tone. "When the politician finished his speech, even the lawyers in the crowd were amazed by his lies."
Used as "until", the word puts emphasis not on the action itself, but on when it's being performed. "The politician spoke, until the lawyers were amazed by his lies."
Bonus: Try wrapping your head around this one, "Until the politician finished his speech, even the lawyers were amazed."