It doesn't really matter how long you've been dreaming. If you are used to doing reality checks (faithfully) throughout the day on a frequent basis, you may do one at any point and become lucid.

I've also found that it's best to do more than one RC just in case the first one fails. As Ezekiel said, I've also found that I'm already half certain that I'm dreaming even before I do an RC. They just help you build confidence that you are, in fact, dreaming.