Robby when I use too make wine it would run for a couple of months. I didn't test the one I made with water sugar and bread yeast but I sure thought it would go longer than a couple hours. If I remember right some batches of wine will stop bubbling and even make vacuum but will kick in and start. I really thought water sugar and yeast would go a month or so. Maybe cause I was making larger batches but even gallon batches ran several months. I'll start mine back up and test how long it bubbles. Sometime a shake or stir will get it going faster.
Here is a mixture that in a 2 liter will go 16 days:
DIY CO2 System for Planted Aquarium
Another update: I wonder if you may have a leak at the cap or bad seal?