OpenAI updated its Help Center article 'Understanding ChatGPT and Codex usage limits' over the weekend. Starting in December, Plus, Pro and Business plans get one usage allowance shared across ChatGPT and Codex, as OpenAI brings Work's agent features into the main ChatGPT experience.
Usage will be measured in tokens. How fast you use it up depends on the model and the task, and OpenAI says more advanced models and complex tasks use more. Chats with Instant models won't draw from the allowance at all.
Part of this pool exists already. OpenAI's page on using Codex with a ChatGPT plan says Codex, ChatGPT Work, ChatGPT for Excel and Workspace Agents share one allowance and credit pool on plans that use one. December widens that to ChatGPT as a whole.
What's missing: a start date inside December, and token numbers for any plan. OpenAI says it expects most people won't hit resets more often than they do now. Combined usage and reset times will show in Settings > Usage, and when you run out you can wait, upgrade or buy credits.