It's a phrase used to ask about the duration of something, especially when inquiring about a time limit or deadline.
Here are some examples:
* ¿Hasta cuándo vas a estar en la ciudad? (Until when are you going to be in the city?)
* ¿Hasta cuándo tengo que pagar la renta? (Until when do I have to pay the rent?)
* No sé hasta cuándo puedo esperar. (I don't know how long I can wait.)
The phrase "hasta cuándo" can also be used in a more informal sense, similar to the English phrase "how long?". For example:
* ¿Hasta cuándo vamos a estar aquí? (How long are we going to be here?)
It's important to note that "hasta cuándo" is a two-word phrase, and should not be confused with "hasta cuando", which means "until when" but is a more formal and less common way of saying it.