An estimation of how big the market for habit apps is:
Habit Streaks downloads for habit and habits keywords: 3,000 per month
According to 2010 study on SEO results share amongst top 10 results:
I started learning to program two years ago. I released my first app, Habit Streaks, shortly afterward. It was a simple habit tracker where 45,000 people created over 80,000 habits with it. The following is what I learned and how it led me to build my third app, Tribes.