No way to do this that I'm aware of out of the box, unless there's an addon that allows it. But I would think twice about it even if you could. You're going to tell users they have 30 minutes to edit their post, unless someone reacts to it first? Be prepared to get conversations (and other users probably too) asking if you can please remove the reaction so they can edit their post - they only have 10 minutes left!! Or asking you to do the edits for them after the fact. Personally, I'd be more than a little annoyed if I took the time to create a long, thoughtful thread, maybe an article, and then couldn't go back in afterward to at least edit it for spelling mistakes and grammar just because someone immediately reacted to it.....