All PADI scuba certifications include independent study as well as in-person instruction.
For the Open Water Course, independent study can be done through watching instructional videos, reading an instruction manual and answering multiple choice knowledge reviews. It can also be done through PADI eLearning, a mutlimedia online learning platform. Both of these methods can take anywhere from 6-10 hours depending upon the student's personal rate of progress, but most people find learning through eLearning to be a lot more enjoyable and quicker.
The in-person instruction with PADI eLearning typically takes 2.5 days. If a student is very confident in the water and able to demonstrate mastery of scuba skills quickly, then it can be shortened to two days - though this is usually the exception and not the rule.
The in-person instruction using the manual & videos, typically takes 3 - 3. 5 full days. The additional time is required because instructors will review the answers to your multiple choice reviews with you in person, and you will also do your required quizzes and exams on-site (vs. online beforehand).