You can write a very good concise opinion on a movie in about 4 or 5 sentences, but if you also want to tell people what it's about and who is in it, you better add another 4 to 10 sentences. And if you want to talk about some of the technical aspects of it, or give some perspective on its relation to other films, then you better plan on adding somewhere between 4 to 12 more sentences. If you have a high movie I.Q. you'll also want to compare a film or write enough about it, that alert readers can compare the film to others of its genre.
Sometimes you want to try and explain why the film resonates with you so strongly. You might be downright passionate about the film. Before you know it... you're well past the 500 word mark and maybe even into the 1,000 or beyond mark.
However if you start getting repititive.. start over-explaining things, or trying to overly justify your opinion, or give too many details about plot machinations... then you're writing a too long winded and boring type of review. Trust your readers. Would you want to read the review you are writing? Would you want to read it anytime of the day?
One thing I never do is spoil important plot points. If I need to discuss them in my review they I will alert readers SPOILER of possible SPOILER.
i also challenge myself to write worthwhile reviews that are not just cookie cutter kind of things that anyone could write. Sometimes I think my reviews are very much worth reading.... other times.... well at least they are informative....
Keep writing, Reading, Good pointing.
Discover a new writer you've never read before. Click on an unfamiliar name and read two to three reviews by that person. There's some very very good and talented or at least FUN writers out there.