I started using Duolingo a month ago. When you are out of hearts you can do "practice" sessions to replenish. Sometimes it gives you the option to watch an ad, but only a single time. My wife bought the Pro plan where you never run out of hearts but I figure if you are out of hearts that probably means it's time to practice instead of doing new material.
The problem with Duolingo is that not everyone sees the same, even under iOS. When Hearts came in, it was immediate on my Wife's account. But I didn't see them for 6+ months. We both regularly update apps too. It is a very strange thing.
