Some advice on film critiques

I dont really know what you mean by how to critique a film "properly". There is no right or wrong way to critique a film its what you see in it personally or how it makes you feel. for instance look at a film review in "the sun" and then look at one in "cahiers du cinema", youll find to very different approaches but to say one is a more valid critique then the other is foolish. As for how long it should be how long is a piece of string? Again Im not sure what your asking(is it for an academic essay, a magazine article, a newspaper review or are you just curious as to how film scholars approach the subject?). One site where I find the reviews and articles very informative is bright lights film journal(google it) or you might want to check out sight and sound which you can get in any easons. In the mean time if you want to be more specific maybe youll get more help.