Ever since Duo Plus was replaced by SuperDuo, there are no more 3-days on the house. Three days of Duo Plus came in handy after 50 days of toiling. This allowed training on / revising errors you had previously made in any of your DL-courses. Single skill tests and legendary challenges were also free of charge.
Sometimes XP peaks during those days of Duo Plus were quite remarkable. But you had to work extra hours to earn those.