With a free Yousician account user has one free lesson available every 12 hours.
Lesson length depends strongly on what the users do in the app. E.g. if a user spends a lot of time navigating through missions, watching tutorials, or practicing while the game is paused, or similar tasks, it does not use up the lesson time as fast as when someone is basically practicing all the time.
In practical terms, free users can thus have a meaningful session of around 20-30 minutes every day. We cannot provide users with a specific number about lesson time, because it varies so much according to the behavior.
Of course, our recommendation is to subscribe to Yousician premium, where user has unlimited playtime, and access to the whole songs catalogue.