Hello. 
Before asking a question always remember to search using the search browser as there could be someone that made the same question before. 
In this post, Evan Lewis (a legend here at KA) answer some time-completion questions on different courses. He's only stating the time it takes to watch the lectures. Then you have to add the time you spend doing the exercises, re-doing them in case of not having proficient, etc. 

As you might know it depends on the person. 
Time Estimate for Course Completion – Khan Academy Help Center

I can't neither find a more complete post I saw before but if Algebra II has almost 13k energy points for mastery, it might take you less than 20 hours to finish viewing all the videos. Good luck. 

Hello, jolshin!

In addition to Pablo's comment, I'd like to clarify that Khan Academy doesn't have estimated completion times for the courses as we believe in self-paced learning. Every learning journey is different, and each learner will spend a different amount of time in the courses they're studying.

I truly appreciate your understanding! Have fun learning at Khan Academy!

Dustin
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