24-hour messaging window
Also called: Standard messaging window, Human Agent tag
The 24-hour messaging window is the period after a person messages an Instagram account in which the account may reply; outside it, standard messages are not allowed.
Three clocks, not one
The standard window: after a person sends a message, the account can reply for 24 hours. Each new message from the person restarts it.
The private reply allowance: one DM to a commenter, within 7 days of the comment. It is a separate rule from the 24-hour window, and it is the one that governs comment-to-DM.
The Human Agent tag: lets a human support agent reply up to 7 days after a person's message, for cases that cannot be solved within 24 hours. It requires App Review and is for people, not automation.
A window is not a rate limit
A window decides whether a message may be sent at all. A rate limit caps how many calls can be made in a period. Sending outside a window is a policy problem even when the account is nowhere near any rate limit.