I honestly never thought about it.
If I were to make a judgment now, it'd be when the new consoles are more readily available, so probably between 6-12 months after launch. It's also when the ramp up in releases for next gen consoles start to pick up, and the current gen slowly begins to fade out.
I guess I find it hard to think of these new consoles as "current gen" when so many gamers aren't able to jump in yet.