I played for the last 3 days on this account and it was maybe about 4 hours each day.
But I always play while doing something else (multitasking), it's not like I'm sitting 4 hours and I only play ;-)
Actually, I could finish this experiment after the first day, when I came to top50. Further play was already less effective and improving the result was getting more difficult. Although I was even 22nd at some point, but I was not able to keep this position.

So one can really make some money by using only rented cards

Yes, I earned about $25 total but it definitely takes time. Suppose I would end the game after 5 hours it would be $5 per hour of play. For some it's a lot, for some it's a little. For me it was just a fun experiment. Maybe I'm going to repeat it end the end of current season but I will try to reduce the total time spent to maybe 2-3 hours (it would be easier because I will start from 3400 points, not from 0 points).