Hello Patti, and welcome to FAA.
Of course, there are many variables that could affect the answer to your questions. Mike has already mentioned many of the primary variables. Some not mentioned may include quality of work and desirability of subject matter by potential buyers. Probably the most important thing would be your passion and talent for marketing your work. Another thing to think about is how your work is found. Usually this will be through someone's searches. How will they search? One way is through search words and phrases. That means you will want to include as many related search "tags" as you can.
No one can really say for sure when your first sale will be. It could be today, or 6 months from now. There are some who take longer and others who catch the eye of an admiring collector, but on average, it takes at least a few months to get something going. Of course, I would recommend to read all you can about the site, how it works, and what they suggest for you to do. You will get some tips from this thread and others already existing if you search for them.
Joining for premium advantages is a good deal when you compare everything and listen to others advice on that. Some of that decision may depend on your enthusiasm, passion for art, and trying to get them sold. If you have a niche', learn how to gear that toward your possible buyers. Build up clientele the best you can which may include linking your galleries of images to social media including Facebook and Twitter. (See info about this "behind the scenes").
Remember that you may have some of the most beautiful flowers, butterflies, and kittens in the world but the market is so saturated with those subjects that it reduces your chances of selling that kind of thing. I personally would focus on something you love, but something that is different than what is already saturated in the market.
All the best to you!