For “How long is the movie?” you’d say “How long does the movie last?” or: “Combien de temps dure le film ?”.

For “How long will we be there for?”, you’d use just like you thought “Combien de temps resterons nous …?”

“Combien de temps“ is right, but “passe-t-il” is not. The appropriate sentence here is “durer” (“to last”) and your sentence should be “Combien de temps ce film dure-t-il ?“ or “Combien de temps dure ce film ?“.

Note that “Combien de temps dure-t-il ce film ?“ is erroneous as well.

Moreover, while “Combien de temps est-ce que nous y resterons ?” is grammatically correct (“Combien de temps y resterons-nous ?” would be better), take care that, in French, you don’t go “to the movie” nor can you be “at the movie”, you go “to the movie theatre“. You have to take care that your “y” can’t refer to “le film” the same way “there” refers to “at the movie”. Your “y” has to refer to “le cinéma“ (“the movie theatre”). So if you say, in sequence:

How long is the movie? How long will we be there for?

you would have to translate it as

Combien de temps dure le film ? Combien de temps resterons-nous au cinéma ?

More details about “passer“ in this context

“Passer” in this context does not accept a complement (it of course does when it means to pass a ball to your striker). Thus, “Le film passe deux heures“ is incorrect and so is “Combien de temps passe-t-il le film ?“. Although “passer” has many meanings, in your case I can think of two:

1. Le temps passe.
   Time goes by.

2. Le film passe dans vingt minutes.
   The movie is shown in twenty minutes.

“Dans combien de temps passe le film ?“ or “Dans combien de temps le film passe-t-il ?“, meaning “How long before the movie is shown?“ are thus both correct.

“Dans combien de temps passe-t-il le film ?“ is not correct as written. Indeed, the subject to “passe” would then be “il” which, by the sentence structure, could not refer to “le film”. A bit like « Is it the movie any good? » is incorrect in English: you can write “Is the movie any good?” or “Is it any good?”, but you can’t use both “it” and “the movie”. If you want to use emphasize “le film”, you could say (but I find it a bit weird) “~ Dans combien de temps passe-t-il, le film ?” (with a comma); I would rather use “Et le film, dans combien de temps passe-t-il ?”.

You can say

Pour combien de temps est le film?