Highly unlikely to improve 140 points in 3 weeks. I've pretty much never heard of that happening. I know that's not what you want to hear, but better to know and make realistic plans.
Most people would need to study for several months in order to achieve that kind of improvement. You're going to have to make a choice between timing (when you take the test) and goal (what score you hope to get). Take more time, have a higher goal. Take less time, have a lower goal.
We don't have stats on average score increase from practice to real b/c (a) practice tests are taken under varying and often non-official conditions, and (b) comparing a practice test to the real thing is comparing apples to oranges - not the same test, not even the same situation (you know the practice test is just practice, you know the real test counts - that stress alone can make a major difference).
In addition, the level that you're at and the level that you want to get to matters. It's a lot easier to go from 500 to 600 than from 600 to 700, even though both of those are 100 point jumps - the higher you go, the harder it gets to keep going, basically.
If you really want that 700, you're likely looking at a bare minimum of 2 months, and probably closer to 3-4 (depending, of course, on the quality and duration of your study).
You may want to check this out:
http://www.manhattangmat.com/blog/index ... tudy-plan/