Hello fai_saudagar,
First of all, it depends on which visa you`ll be applying.
J-1 visa: valid for two years, and gives you the right get a paid work related to the area of your education. The validity of the visa is considering the training and instructing time, so, the faster you finish you training, the more you`ll have time to instruct.
M-1 visa: usually valid for two years. For every 90 days studying you`ll get the right to work for 30 days, meaning that if your training take a year, you`ll be able to legally work as a instructor for 4 months.
In both cases, there`s no way to be 100% sure that you`ll get the job at the end of your training. The best thing to do is to go to a school that treat you with honesty since your application process. Don`t believe in any school that assure you`ll get the FI position, you better stuck to a FTO that honestly tell you that it depends on your development, your skills and the availability of positions... More likely these schools will be looking at you carefully, and will be willing to give you an opportunity, and not only make an offer at the beginning that they`re not sure they`ll fulfill in the end, just to get your money.
wish you the best, and good luck!