You can learn the basics of Java in a day. Read our tutorial Learn Java in a day.
One you are familiar with the basics of Java should should start learning more advanced topics from Java tutorials section.
Usually it takes around 3 months to master the advanced concepts of Java. But it depends how much effort you are putting towards learning Java programming language.
After learning Java you should learn the advance topics which is used in developing the Enterprise applications.
You should check Frameworks tutorial section for more details on the advanced Java programming technologies.
Happy learning Java!!!