I agree with bb's post, above, regarding the daily workload. I personally found that the effort/reward equation only worked in my favor until my fourth or fifth hour - which is to say that I was entirely too exhausted to actually learn anything after I hit the 4hr mark. It's really good to work for long stretches in order to build stamina (i.e. solving quant problems for 60-90 minutes, continually, followed by 60-90 of verbal) but anything more is really beyond the scope of the GMAT. Of course, only you can determine at what point you're increasing your stamina and at what point you've overshot your mental sweet-spot.
A couple of things about your experience thus far:
1. It's a little strange that you received a rating of "Average" in both of your quant diagnostics, and followed that up with a 35 in your first practice CAT. Similarly, being "Above Average" in both RC and CR indicates that you should be scoring 30-35 in your V fairly easily. I would recommend working on the psychological aspect of the exam - either you were nervous (consciously or subconsciously) when you took your CAT, or else you had a bad day, or you became fatigued from doing a test as long as the GMAT. In all likelihood, you will see some improvement in these categories as you do more and more practice problems and find your groove. I scored "Average" in both of my Q sections on the diagnostic on OG13 and wound up with a Q49 on the real thing - mostly due to the building up of stamina and becoming familiar with the different topics that are covered (more on that below).
2. SC is difficult. Even for native speakers, it can be absolutely horrible, because the colloquial English language (wherever you may happen to speak it - whether it be the UK, US, or any other part of the world) is FULL of ambiguities and mis-expressions. If you are good with grammar and the parts of speech, you should look at the MGMAT books
(there is a Sentence Correction one) and see if it can help. On the most atomic level, SC is about the rules of grammar and idioms. If you know the grammar, you have won 75% of the battle. The good news is that RC and CR are significantly harder to learn in a short amount of time - so, being Above Average in those is to your benefit (although you shouldn't get complacent and not review these topics, even if you feel that you are good at them).
Ok - continuing the first point I made above:
I like to characterize the Q section in the following way - it is made up of two parts. The first (and most important) part is the fundamentals of mathematics. That is to say - are you reasonably good at translating word problems into mathematical expressions? If given a geometric diagram, can you fill the missing pieces using the geometric axioms? Can you do basic probability and statistics? If the (honest) answer to those questions is "yes", then you may want to work on strategy more than the actual mathematical operations (part 2, below). If the answer to the above questions is "no" (and if the answer is "I'm not sure", it should be treated as a "no") then that's no problem. There are plenty of books (my favorites have always been MGMAT, but I'm sure there are other quality publications out there) that can teach you the fundamentals. It won't take long - oftentimes, all you need is a refresher. Nevertheless, learning the fundamentals really well should guarantee your a score of 35-40 on Q.
The second part of Q is the refinement of your thought. There are a lot of smart people out there who have no idea how to begin solving for a math problem. You need to develop your thinking to the point where you group and sort information mentally, foregoing the process of manual calculation (long multiplication or division) in favor of simplifying the expressions and formulas on your piece of paper to come up with easy answers. In my mind, that golden 40-51 scale on Quant is nothing more than a measure of the quality of your mental work between the moment you read the problem and the moment you start performing actual calculations.
As for taking an online class? It all depends on how you learn best. If you enjoy having an instructor and a more structured setting, then you should go for it. If you're more of a free spirit that enjoys following a less conventional learning pattern, it may be best for you to do without the official class. In any case, I would recommend the MGMAT books
, and every OG13 practice problem you can tackle.
And yes - I'm convinced that with a combination of hard work and smart work, you will obtain your goal of 600!
Good luck to you!