If you intend to go into SFX/Prosthetic/Appliance Makeup you are looking at bout 3 plus months in school. Of course that is dependent upon the school you choose, and their curriculum time frame. That being said, of all of the different types of 'makeup artists' out there, SFX artists' will always work. That type of work is ALWAYS in demand in this industry.
Once you graduate school, even with SFX, you will be doing free work for a spell. It's how you get your name out there and build contacts. If you want to work for a studio, that takes a bit more time and effort. All of us in this industry are grouped into Unions, and if you want to get into the Union, and be a rostered artist, you will have to have accrued a certain amount of hours,and you will have to pass all of the Union tests and pay a very, very large fee to get in, and the more work you do, the higher up the Union ladder you will get. You will be doing individual projects to be able to get into the Union and work with studios and production companies.
Depending upon where you live, you get 'freelance' jobs by trolling the industry papers or various online sites wherein jobs for SFX artists are listed (Mandy.com, Production Hub.com and yes, even craigslist.com.) You have to find the work, it won't come to you, but if you are persistent, you will succeed. I wish you the best of luck!
I'm a Film/Television Makeup Artist