Ahh, the elusive backstory. We spend countless hours filling in the holes in our characters lives so that they become multi-dimensional, and then we go to write the book and are told not to put that information in. It's true though - how do you like reading info dumps?
So how do you know when it's time to put the info in? And how do you know how far to go?
I'd say the answer is deceptively simple. Put in only the information necessary in order to fully appreciate the story you are telling. And do it when the information is needed in order to go any further.
If you withhold the backstory until the last moment, you are creating tension. But if you withhold it to the point where things are unclear and frustrating for the reader, instead of an intriguing mystery, you've withheld too much for too long.
Example time! We must go with Harry Potter for this one. (I will focus on one part in particular, but the wonderful blogger SP Sipal had a fantastic post on JKR and how she handled the backstory of this portion of HP if you want to read more on this)
In the beginning of book one we learn that Hagrid has borrowed the flying motorcycle from young Sirius Black. We need this info to understand how it is that Hagrid arrives as he does on Privet Drive with baby Harry. But JKR doesn't fill us in with all the details of who Sirius is, or why he would have been at the crime scene. And in fact, we don't give it a second thought, do we? We are far too concerned with the enchanting trio of Dumbledore, Hagrid, and McGonagall, who Harry Potter is, why he is special, and what on earth it has to do with these horrible Dursleys.
But come the third book we meet Sirius again. And throughout the book, at the right moments, JKR reveals the story to Harry, and vicariously, to us. Throughout the series, we learn enough to be satisfied, and yet, just like Harry, don't have all the important information until the end. In fact, as much as we learn, we never know everything JKR does. And that magical balance is precisely what we have to aim for in our own work.