I've been using the 1M window at work through our enterprise plan as I'm beginning to adopt AI in my development workflow (via Cline). It seems to have been holding up pretty well until about 700k+. Sometimes it would continue to do okay past that, sometimes it started getting a bit dumb around there.

(Note that I'm using it in more of a hands-on pair-programming mode, and not in a fully-automated vibecoding mode.)