After you get your law degree, how difficult is it to become a successful criminal defense attorney?

Trying cases  is not the easy posturing you see on TV and the movies. It takes skill and experience to formulate a question, reformulate it on the spot  to respond to an objection, to master all ( and I do mean all) the rules of evidence and procedure, to have a strategy  and the ability to speak to make your record for appeal and keep all the evidence straight.( you laid the foundation but did you remember to actually offer that thing into evidence?) Even if you took trial classes in school, this is not something you do easily or well right out of the gate. In addition, when defending a criminal you are pitting your knowledge and skill against a prosecutor  for stakes which are absolutely irredeemable . If you screw up, it is a hunk of your client's life which he will spend  in jail. He can never get that time back and it is largely based on your skill and preparation. Don't forget that your referrals come from past clients, more so from the ones walking free, not the one's in the joint. To answer your question, in order to become a successful criminal defense attorney takes a great deal of time  and experience in the trenches. And then there is the guy you get off on a charge of sexual assault and aggravated battery  of a minor who turns around and rapes a 4 year old. Sleep tight ,counselor, you have a hearing on a someone's 4th DUI at 9 and they are bringing in the toxicologist to testify.

Depends on the country, but I don't think it would by easy anywhere in the world. In most common law jurisdictions, just being aware of a bunch of statutes, cases and other laws is what usually gets you a degree - but when it comes to courtroom practice, you'd be surprised at the amount of work, social flair, networking and other 'soft skills' that are required to get anywhere as a pleader. Depending on how fast you learn, it would take you anywhere between 2-5 years to make a name as a criminal defense attorney (or defence counsel, depending on the jurisdiction).