I wouldn't wait for a "formal opening" before applying. In fact, if you do, you're competing with even more people.

How has that worked for you?

So when do you apply?

Immediately. There's no harm, and it's more likely that a company is always interested in good applicants. You can always follow up again when the position does show up on their open position listing.

Also, if you apply during the "offseason", it's more likely that technical hire has more time to respond to you in more detail. I've developed a couple relationships that way that paved the way for an eventual hire in one case and a technical relationship in another.

So you just submit your resume to places you'd like to work, even if they're not hiring? Do you even look at job postings? That's interesting...