ChatGPT and Codex will share one usage allowance from December, OpenAI says

TL;DR

OpenAI's Help Center says Plus, Pro and Business plans will share one usage allowance across ChatGPT and Codex starting in December, measured in tokens.
Instant chat won't draw from it, and you'll see your combined usage under Settings, then Usage.
Notebookcheck notes the article gives no start date, no allowance sizes and no reset schedule.