Final answer:
The time to master a skill varies per individual and ability. Mastery should be assessed by consistent high-level performance, not a set number of days. Instructors should mark skills as mastered on an aqua-card only after thorough assessment.
Step-by-step explanation:
The question "how many days does a student need to master a skill before the instructor should fill in the check mark on the aqua-card?" does not have a universally applicable answer, as the time needed to master a skill can vary significantly from one student to another and depending on the complexity of the skill itself. Generally, mastery implies a high level of proficiency and should be determined by consistent performance over time rather than a specific number of days. For this reason, it is important for instructors to assess students individually and ensure that a skill is consistently performed at a high level before marking it as mastered on an aqua-card or any kind of tracking system.