I found that my Qbank scores took a sudden jump (from an average of ~50-60% to ~65%) after I finished about 1000 questions. That's in line with what a lot of people say online... apparently, it's a common phenomenon. What I didn't expect was that my scores took another sudden jump (up to about 75%) after I finished about 3000 questions. I think it's because after you finish a certain magic number of questions, you get to the point where you've seen all of the common things that are asked.