How much time (approx) can it take to learn and use hadoop ?

I wanted to learn hadoop as soon as possible so that I can work on enterprise level projects. Given that I have a "pretty good" knowledge of Java (OO design, Threads ,Data structures, Generics), how much time do i need to get a decent foundation in hadoop ? Is it like studying for the OCJP exam, which most people can prepare for in just 3-4 months ?

Assuming your study style is a mix of reading and doing, I'd say as much time as it takes for you to go through a decent chunk of a good book on Hadoop would be the investment in just Hadoop itself. There a lot of subprojects of Hadoop though and depending on the enterprise, they could also be using some of those to make it easier to work with Hadoop. Just as the Spring Framework has many different parts to it, learning everything about Hadoop is almost a herculean task. But talking about just the basics and getting to a level where you can be effective in an enterprise setting, you're probably looking at 3 to 6 months of serious play-around time.

Caveat: Everyone has their own definition of "can be effective in an enterprise setting." I set the bar pretty high, so your mileage may vary.

In addition to what Junilu have said above, going throug the book "Hadoop: The Definitive Guide" by Tom White should give you a fair idea on what you need to learn. A typical use case of Hadoop involves the use of Hive and/or Pig (if Hadoop MR is not sufficient) to process the data stored in HDFS. If you need random read/write access to your data stored in HDFS then you also need to look into HBase.