Complete Java programming take around 6 months assuming 4 hours learning per day.
20 days is good to learn Core Java and Advanced Java.
You can many spend 3 hours in morning and 3 hours in evening to learn Java. And in 20 days you can learn many topics.
Start with the New to Java? to find the learning path.
Then start with the Java installation: Installing JDK 8 on Windows 10 and configuring the settings
After this you can start developing your programs.
You can check following tutorials of Core Java: