I did my CFI with ATP, it is a good program if you are a very good self study. They say it takes about a week. I went to jax to do mine, one week of ground plus sim and after that my ride was scheduled a week later
So I had two weeks to study. As far as the gaurunteed job. It really depends on how busy they are at the time you go in, I have friends who went and were sent to some no name town and stuck on permanent ground instruction because garunteeing everyone a job at some point the instructor student ratio is going to be unfavorable. That being said I know guys who went in and instructed for a year and got close to 1000 hours. The pay is not the greatest either, when I went through they offered $23 an hour which for a CFI is kinda low. The club I instruct at now the minimum you can charge is $45. So if you don't mind moving somewhere you'll probably not enjoy to possibly get stuck being a ground instructor for a bit before logging some serious hours I would say do it.