I study 20 hours per week, which means 4 hours per day, 5 days a week.

I try to do different things to avoid "the same old".

I only check on grammar and take some lessons twice a week.

I do believe that you have to expose yourself more by listening, talking, reading etc, it's much better than spending 3 hours in a row studying to close your book and go to sleep.

If you are young and intelligent, you might spend 2 hours in a week to learn a new subject, but if you are older than 30, you may need much more time over the learning, possibly more than 10 hours in a week regularly. 

Furthermore, you had better view what you have in your hands. Supposing you are not having good comprehension in basic wording, grammar and sentences, in that case, I would recommend you spend more time on the basic lessons to acquire the fundamental building blocks to make your practice more efficient.  However, if you have sufficient knowledge in English expression, you may save more time to practice with your learned knowledge. 

So far as I am concerned, I am convinced that learning a language is a journey without destination so I just keep on learning, thinking and practicing all along. 

Not much to add to Susan's excellent post. I would just emphasize that quality of the time spent and consistency of practice are both key. I don't believe there's one definitive answer to your question -- I'd encourage you to experiment with the two scenarios you described and see which works better for you. As Susan pointed out, taking lessons when you're exhausted just to take them is likely of little value. It's good to take the amount (and length) of lessons that you're able to fit into your life and be actively engaged with. It's good to have a variety of activities and strategies, including lessons, that allow you to practice every day, in different contexts and at different energy levels, so that you can get the most out of each one.

Cheers,

Trevor