No way of knowing I don't think, but if I had to guess by the middle of May for certain. Possibly earlier. That said that's just speculation. You could always try some other games? There's some really nifty stuff on the site, both made with HTML and not.
Patreon’s most recent version says that enough progress was made to maybe start working on morning scenes, so I imagine sometime soon(ish), maybe before the end of the month. Starting a new day usually seems like a good place to walk away and go back to working on Power of Will for a bit, as far as I can tell with G28’s usual patterns.
But obviously don’t quote me on that or get any big expectations. He works at his own pace, and the game’s all the better for it thanks to the time and care spent.