Keep the clock running after 25 mins without notifying. The longer the focus, the bigger the building at the end!

As a way to provide variable rewards, and also not distract someone who is in deep focus after 25 mins (because as developers, you know these spans can be much longer) keep the timer running and only stop it once the user gets distracted or selects to take a break. I haven't looked into your building generation algo but you can maybe build bigger buildings for longer focus times! if someone doesn't get distracted for a whole day, BOOM here's a burj Khalifa in your city