Yeah, I can confirm that the landing pages said 24 months, but the application pages said 18 months yesterday morning. And as of this morning, both the landing page and application page on the "other cards you may be interested in" on the Citi Pre-Qualified site show 24 months.
If this is going to be the new norm, I'll definitely have to consider whether I want to wait much longer to try for card #4. Even if I don't get the bonus, I can cancel the card immediately and just play the new waiting game from then.