A ChatGPT account. Locked chats are tied to your account, so signing in is required. They come with Free, Go, Plus, Pro, Business, Enterprise, and Edu. No paid plan needed.
Nothing to install. It works in the browser and in ChatGPT's apps. On your phone, Face ID or your fingerprint can open the lock too.
About 4 minutes.
One timing note: OpenAI is still rolling this out account by account. If Lock chat isn't in your chat's menu yet, it hasn't reached you. It hadn't reached our test account when we last checked.