is it worth it?
Currently we have so much work that barely can keep up, and our salesman only works half time (yes, we are a company of about 10 people, but if you are doing it alone, you'll have enough work and if you dont undercut yourself, you will have enough -and more- money as well).
How long does it take?
It depends, but as James said, if you are doing good work, you will get the right contacts. First it is hard, I would say (alert!! generalization coming) about a year from zero to good money inflow/profit, less if you already have a strong portfolio.
Where to start?
Again, as James said, first stair. I would say a strong portfolio, learn from the best. This and other forums like vray and corona and a lot more are out there that are good, but watch out not to take on challenges that are too big or which you dont understand at all (like for e.g. material scanning) because it will take a lot of time to learn and maybe add just a little to the overall quality of your images, i would say learn about materials overall and composition/lighting. Learn to watch reference, that is one of the most important skills in my opinion and pay attention to details BUT reference reference and reference. Learn to see the details and get as much out of the reference images as much you can (almost everything, materials, form, details, composition, reflections, the size of dust particles that shine as the sunrays hit them through the window etc).
low paying sites
The people you are looking for, are not looking at those sites, they want someone reliable and who does good quality (you need to be that one though).
There are companies that do 3D that are looking for artist to give work to, so you can look at those too, not just people who need an image for their marketing campaign.