How long has this Q&A feature been around?

As Micheal said, at night between the 6th and the 7th, as for the 18 days 前 thing, it is due to secret testing, I imagine, they did the same with quizzes, where some users tested it out, but it was only properly released at night between the 6th and the 7th.

The ファンポップ 回答 feature was officially launched in the middle of the night spanning March 6 and 7.