Length: 2,200 characters
A comment can be up to 2,200 characters, the same as a caption. Too many hashtags or mentions in one comment can make it fail to post.
Speed: no published number
Instagram doesn't publish a daily comment limit. Commenting very fast, or posting the same text again and again, can get you action blocked for a while. That's also how Instagram spots comment bots.
What this means for bought comments
Bot comments hit these limits and patterns constantly, which is why they get removed. Real comments, written one by one by real people, don't.
Questions
How long does an action block last?
Usually from a few hours to a few days.
Can I comment with only emojis?
Yes, but a few words get more replies.