Hi Remiss! Four acts were planned from the start, as was the general layout of the story. If we wanted to finish fast we wouldn't have added the whole section about Karl's mansion and the game would probably be done by now.
The only thing pushing me to end the game is that my artist is very enthusiastic about drawing Luna, the wolfgirl that was also posted in here a while ago.
Saying to 'end' Sana's game sounds wrong though. We can always return later to add more scenes, for an extra act (I got a great idea for a prequel chapter) or a straight up sequel game where you... oh wait, spoilers. At least that's how I see it currently.