How many words should my college admission essay be?

I’m writing my college application essay, and through the Common Application, the essay does not have a maximum number of words, only a minimum of 250. I will be applying to a few state schools and a few higher private universities. How long do you think it should be? I want to stay close to 500 words, or around two double-spaced pages, but how far can I stray from that? How long do you think it should be?