Let me add some thoughts, in response the Andy's request. I have responded to this same questions many times to students taking the AuthorAID MOOC, in fact. For a start, I would recommend that you start reading widely around your area of interest. Start with some recent review articles, and follow the reference trail from there. That is, when you see a reference to something interesting or relevant, check out that reference. Keep doing this until you get the feeling that all new papers you find tell you pretty much the same thing, and you are not gaining (m)any new insights anymore. At that point, you have reached a saturation level -- and at the same time you have gained a great overview of the state of your field. There is no hard and fast rule as to how many papers one should read, or how far back to go -- after all, some of the most seminal papers in your field may have been written 50 years ago. They may not be fully correct anymore, but they most likely represented the basis of the field, so it would be advisable to know about them. Therefore, I normally object to people saying that you should just focus on the last 5 or 10 years -- the most recent papers will tell you the current state of the field, but by not going back further, you are missing out on your field's foundations. I hope that this helps!