I believe that the best way to use Qbank is to always do questions randomly, with no repeats, in 50 question blocks. Filtering by subject or system won't help you in the end when you're trying to practice as much as you can because you'll run out of questions in certain subjects before others.
When I was studying last year, I had more of a graded approach to the number of questions I did. I did questions on a regular basis all throughout, but definitely focused more on questions towards the end. At the beginning, I did 1 block every couple of days. By halfway, I was doing 1 block a day. By 3/4 of the way, it was at least 2 blocks everyday. By the last week of reviewing, I was only doing questions all day long. I got through all of Qbank and IV Qbank during my 7 week study schedule.
Don't rely too heavily on Qbank, though. While it is a great source of questions, it can focus you in the wrong directions.