Instagram won't let you DM someone whenever you like. Each conversation has a messaging window, and an automated DM can only send while that window is open. Here's how the windows work so your replies always land.

The two windows

• 24-hour DM window. When someone sends you a DM, a 24-hour window opens. You can send automated replies any time inside it. Every new message from that person re-anchors the window — the clock restarts from their latest message, so an active back-and-forth stays open. A reply to one of your stories counts here: Instagram delivers it as a DM, so it opens the same 24-hour window and re-anchors it like any other message.
• 7-day comment window. When someone comments on your post, you have 7 days to send them a DM in response to that comment.

What "expired" means

If the window closes before your DM sends — the person stopped replying and 24 hours passed, or 7 days elapsed since the comment — the send is expired. big marks that automation run as expired and stops trying. It does not queue the message to fire later, because Instagram would reject it. See Run statuses for how expired relates to the other four.

How to stay inside the window

• Reply fast. Automations fire within moments of the trigger, so you're normally well inside the window — this matters most if a send was delayed (for example by a plan cap).
• Design opt-ins around fresh activity. A comment-to-DM automation on a new post fires while the 7-day window is wide open.
• Don't expect to reach cold conversations. big can't reopen a window that has expired; the person has to message or comment again first.

Next step

If a DM didn't arrive, check whether the window had expired in "Why didn't my automation fire?".