I think it depends on the professor as far as how soon they want you to contact them. And of course, since you don't know them, it's hard to know when they want you to contact them! I applied at Wheaton this past year (for admission fall 2013) and I actually emailed and met with my prospective mentor there a year and half before the application was due. And I was not the first person to do that, and the prof said he loves when students reach out early on like that. It lets him know that they actually want to study with him and aren't just trying to go anywhere. (In the end I made it to the interview round but wasn't admitted.) But then I also applied at Princeton and their website said that they like students to visit in Sept-Nov of their application year, so that's what I did. (Not admitted there either). I think if you're ready to email a prof, it doesn't hurt to go ahead and do it. Waiting too late might hurt you, but I doubt emailing too early would!