I started applying 2 years prior to my separation to every airline that was accepting applications. Bottom line is it doesn't hurt to apply early, just don't get discouraged because more than likely you won't hear anything until you are inside the 6 month window.

In my opinion, the main reason to apply early is to guarantee that you can take advantage of an airlines application window which can open and close without notice. Typically, once you have applied, you can keep your application active indefinitely with consistent updates.