Instagram's 24-hour messaging window
Why you can only reply to someone within 24 hours of their last message, and how this affects automations and the inbox.
Instagram limits when a business account can message people, to protect them from spam. These rules come from Instagram and apply to every tool that uses its official API, not just SyncRelay.
The rule
You can reply to a person for 24 hours after their last message to you. Each time they message you, the 24 hours start again. After the window closes, you have to wait until they message you again.
How it affects the DM inbox (Pro)
When the window has closed, the reply box in Messages is turned off and you'll see:
Instagram only allows replies within 24 hours of this person's last message. You can reply again after they message you.
The banner also shows when their last message arrived. As soon as they write to you again, you can reply.
How it affects automations
The first DM from an automation is a reply to the person's comment, which Instagram allows even though they haven't messaged you yet. When they tap a button in that DM (for example Send me the link), that counts as a message from them and opens the 24-hour window for the rest of the conversation.
That's why an opening message with a button is useful: it starts the conversation the right way.
Sending by hand from Comment Tracking
You can use Send DM for a comment only while the comment is less than 24 hours old. See Why didn't my DM send? for the other conditions.