I train 5-6 times per week (~7-10 hours) plus another 4-5 sessions of boxing/ kickboxing, and see significant, rapid improvement. I'm pretty new, though (~13 months in), so I'm pretty far to the left of the learning curve. When I begin to feel mentally or physically exhausted or am injured, I dial it back to 1-2 sessions per week and still see some progress.
I think Holt hit the nail on the head earlier in the tread- it's really individualistic and all of us need to find that sweet spot of maximizing improvement versus overtraining or sabotaging other life commitments.
FWIW- I enjoy the "life coaching" discussions that develop frequently. Seeing the interactions between the naive stupidity of youth versus the realistic cynical wisdom of advanced age is endlessly entertaining.
My contribution to that discussion- it's entirely possible to chase the "bjj dream" if you understand the realities of life. I could train even more than I already do if I could handle it physically, but I'm at my own "sweet spot." I can do what I do despite having a wife and three kids and being somewhat old for a grappler (38.)
How?
I set up my life to pursue that which interests me... and bjj/mma is my current interest. My wife and I both have graduate degrees and a wide range of practical, marketable skills across multiple disciplines. We formerly held typical white collar jobs and had a "normal" suburban lifestyle. We decided it wasn't for us, craved adventure, wanted our kids to see the diversity of our country, and quit. We were able to do that because we both had the desire and the skill set to make decent money. We spent a few years traveling around the US with our kids making a living off barefoot running and ultrarunning (another sport where full-time "professionals" barely earn enough to afford ramen.)
We eventually settled in So. Cal. mostly because we fell in love with our gym and training (my wife started first.) Now we train together three times per week and treat it as a date. I make most of my income writing books, but have a few other minor, diverse income streams. We're not rich, but make enough to continually pad our savings and retirement funds. We spend tons of time together, I spend enough time with my kids to be relieved when they go to school/daycare, and I have enough free time to dick around with things like f12.
For me, bjj and mma is an interesting learning experience. I'm too old to dream of anything resembling a world champion, but I do train often and compete because the process of learning and improving is enjoyable. I'm entertaining the possibility of at least one amateur fight just to experience it. I'd like to teach this stuff some day, which is a major motivator to learn as quickly as I can. That may involve opening our own gym, but we have extensive business experience and have worked in many facets of the fitness industry for years.
My long-winded point- if you're really serious about making bjj your life, take the advice offered by the more experienced folks. Don't rely on your mat skills to make a living. Go to college, learn a trade, develop income streams unrelated to bjj... whatever. I strongly believe everyone should follow their passion and surround themselves with supportive people that will foster that passion. That life doesn't automatically happen, though. You have to build it.