I'm not one for long posts, but would like to offer my input :-). Learning a language, especially Chinese takes time. For many of us (myself included) the pursuit of fluency is a lifetime journey of continual learning, improvement, and practice.

Hitting the wall is normal. I've hit the wall more times than I care to remember over the last 10+ years of off and on study. I think all learners of Chinese hit the wall on several occasions throughout the journey. There is an excellent piece written by a good friend of mine that provides great insight and humor to what western learners of Chinese experience. http://pinyin.info/readings/texts/moser.html/

It sounds like you've made fantastic progress in only 6 months. My thought is that 4-6 hour sessions are going to be challenging to really absorb much past the first hour. Now, if your just looking for conversation and listening practice that might be fine, but to really try and assimilate 4-6 hours of content would be a daunting task. I used to think that watching lots of Chinese TV would help me improve. Well it depends if you are actively or passively watching. Just sitting in front of the TV wont do a whole lot (from my experience). However, if you watch something on DVD and can stop, pause, rewind, look up the new words/phrases, go back and see how it was used, etc. you might get more out of it (at least I did). But as you can see this is pretty time consuming. If you are watching for entertainment purposes this wont be much fun, but try spending 30-45 minutes doing this and you might be surprised with what you have learned.

Maybe Echo or one of the other teachers might have some good advice. I tend to think that you'd get more out of a 1-2 hour session with a tutor, take good notes, record the sessions, then spend several hours reviewing and absorbing that content, then taking those new words/phrases on a test drive with native speakers.

I think sometimes less can be more if you spend time in the right places, and there is a limit to what can be absorbed and retained over time from marathon sessions.

Hope that helps and best of luck with your studies.