Well it will be wrong to say exactly how much time you should learn Python to become a Data Scientist. For becoming a Data Scientist we should have the knowledge of many other topics too . Python is not the only Programming language used in Data Science. But Python is one of the most important topic for Data Scientist. when you work on Python for about 2-3 years and build good experience then you can go for Data Scientist. We should have the proper knowledge of Python which we can acquire through proper Courses .