The answer to the first part of your question, “…what’s possible”, is increased traffic (which hopefully convert to customers), but without an audit or consultation with an SEO professional no one is going to be able to answer the rest of your questions as there are too many variables to consider. How many competitors do you have? How big is your target market? What’s the current state of your online presence? etc etc. It could take several months to a year to start seeing results.
So my advice would be, find an SEO professional you can talk with for an hour or so, and hopefully, you’ll come out of that conversation with a better understanding of whether or not an investment in SEO is worth it for you.