At what point do you decide to jump into research?
As aeismail says, there's no single answer. As with most things in life, it's hard to know when you're "ready" to do something. The best way is to jump in when you feel like you don't need to prep any more, and then be prepared to shore up weaknesses as you spot them.  

Over time, you'll learn how to prepare yourself to embark on a new research topic: some people like to start solving a problem first, and then return to the literature when they get stuck. Some people like to read a few key papers to get a sense of the main open problems and techniques, and then go off and play. 

The only important thing is that you do something, and not just wait on the sidelines for a mythical feeling of "readiness" to emerge. Be fearless, like a 2 year old who doesn't know enough to be afraid :)