How long is a lesson?
This is completely up to you. The more you learn daily, the faster you will progress.
Daily Goal
The app lets you define your daily goal, starting from 1 minute per day.
However, you should bear in mind that it is very important to train the language regularly. Ten minutes every day is far more effective than one hour once a week, even though both require about the same amount of time from you. So you should set yourself a reasonable goal.
If you still feel like learning after you have reached your daily goal, then of course you can continue learning as long as you want. Many people have daily goals of 5, 10 or 20 minutes, but occasionally (e.g. on a Sunday) spend up to an hour on the app, to really deepen their learning.
Set yourself a reasonable goal. Ten minutes per day is more effective than one hour once per week!
You can always change your goal in the settings.