The answer to your question really depends on several different factors;
Where you live and the type of market you have. (Las Vegas vs/ Kansas City)
Also consider the level of competition in your area. I first received my license in Phoenix, AZ when the market was great. At that time it seemed like every other person you met had a license. To become a Listing Agent took a great deal of marketing whereas, becoming a Buyer's Agent, although very time consuming was fairly easy as a starting point.
Also consider the different paths being an agent offers; Property Management, Rental, Investors, New Build and Development. Consider being the best at one of these. I have two family members (Daughter and Niece) who I gave the same advise. One of them is doing very well in Salt Lake City with Property Management. The other is building a good portfolio of constant "Flippers" (Investors).
One more thing. Use your resources. Once you have obtained your license and found a place to hang it, find a mentor. Most agents I have known are very willing to pass on knowledge and help those wanting to get into the field. The information they can provide regarding your local market and the direction it is headed is invaluable and will probably help you with your direction.
So, with all that being said, obtaining your license is the easy part. I went to the Scottsdale School of Business and Real Estate. If my memory is correct I think it took me about 2 weeks. Researching where to hang your license, determining your direction and building the clientele I would give a minimum of a year.
Like any business venture, real estate requires constant monitoring and effort. Your dedication, effort and tenacity will determine your success.
Hope this helps,