Every language has a skill tree that maps out your course. You begin learning the basics; as you progress, the skills build on each other and become more complex. As a student successfully progresses through the tree, they unlock the next lessons and units. A student cannot advance unless they have completed the previous skills. The goal is to get through all skills in the tree. (And master a language!)
Teachers who use Duolingo for Schools are able to assign skills or XP goals to students to ensure consistent practice. However, Duolingo does not track the time a student spends practicing. The duration of time it takes a student to complete a skill depends on their understanding and mastery of the vocabulary.
For more information on how educators have used Duolingo for Schools in their classroom, visit the Learning Hub or download a PDF of the Duolingo for Schools: Guide for Leaders in Education.