Quick answer
"When" is an adverb which is often translated as "cuándo", and "how long" is a phrase which is often translated as "cuánto". Learn more about the difference between "when" and "how long" below.
when(
wehn
)
An adverb is a word that describes a verb, an adjective, or other adverbs (e.g. to run quickly, very tired).
1. (in questions)
a. cuándo
When was the last time you cried?¿Cuándo fue la última vez que lloraste?
A conjunction is a word that connects words, phrases, clauses, or sentences (e.g. The cat and the dog slept.).
2. (relative)
a. cuando
I was sleeping when he got home.Estaba durmiendo cuando llegó a casa.
b. en que
That was the year when we bought our first apartment.Ese fue el año en que compramos nuestro primer apartamento.
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how long(
hau
 
lang
)
A phrase is a group of words commonly used together (e.g once upon a time).
phrase
1. (related to time)
a. cuánto
Do you know how long we have to wait for the movie to begin?¿Sabes cuánto tenemos que esperar para que comience la película?
b. cuánto tiempo
How long does the movie last?¿Cuánto tiempo dura la película?
I don't care how long you have to wait!¡No me importa cuánto tiempo tengas que esperar!
2. (related to length)
a. cuánto mide
How long is your surfboard?¿Cuánto mide tu tabla de surf?
You need to check how long the table is before you buy a tablecloth.Tienes que comprobar cuánto mide la mesa antes de comprar un mantel.
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