I agree with the others- it depends on you, what you are writing about, how much helpful information you have to say, and what you want to come out of the review.

I have short reviews and I have very long detailed reviews both.  Some products there is not much you can say about, and others there are a lot of questions people may have.

Formula is a good idea like SpokaneMan suggested too.  That helps.

And- not spoiling movies is another.

A good philosophy to adopt is writing a review like YOU would want to read if you needed help on something or want to make a decision to see a movie or something. 

It will come in time ;D