My opinion is that the improvement is somewhat logarithmic, in that at the start, you'll improve quite rapidly, because you're learning "high-yield" techniques. For example, in Section 1, I learnt how to use tables to solve a particular type of problem and that was "high-yield" because there'd always be 2-3 questions which I could use that method for. A few other methods like that and basically my score went from around 28-29 to around 32-33/44. From then on, however, the improvement levels out because you'll still be learning, but what you're learning now isn't as "high-yield". So it probably took me a lot of time to go from 33/44 to around 36/44 (consistently).
The tough thing about the UMAT is the way you have to approach it. In most exams, you can either:
1) Maximise marks gained
2) Minimise marks lost
With VCE, you know you're travelling well when you're in category 2. However, with the UMAT, no matter how good you are, you'll always be in category 1, you just have to maximise your marks, there's just no way you can answer everything and get everything right, so basically you have to be tactical about how you approach it, for example, sacrificing accuracy for speed or skipping questions which you feel you can't do...etc.