That's not what I meant. I mean what going to cause her to become Yandere.
Here's my guess of what will happen, after virgin-kun assumedly tells gal he loves her 100x:
Gal says she'll think about it, then proceeds to tease him again the next day like usual, as if the confession never happened. Sure, she may be riding the high from being confessed to, but doesn't take into account virgin-kun's feelings of making him wait.
Virgin-kun takes 'she'll think about it' and the actions as I previously stated as a rejection (which may be for the best if she's not even taking it seriously), and seeks to find new girl and be friendly with her. How fast that takes is arguable.
It's on the day when gal becomes a yandere that she finds virgin-kun getting chummy with a new girl that she'll snap.