If you're talking about the second trophy, I believe it's when people like your message, not you liking theirs. It just depends on the trophy and what the description says.
I think the fact this forum has only been open 24hours ish, there are less than 300 members but 2000 post indicates a lot of people are posting around 100 posts a day... lol
I would suggest making it admin changeable, it can then be 1, 2, 3, 4, 24, 48 etc depending on server resources and admin choice.