Yes, CJO is good for 12 months. After that, United could ask you to re-interview. When I received my CJO in 2013, I was concerned about having to re-interview because I was in a pool of over 200. At that time United was only taking between 2-6 candidates per class. I ended up going to class in June of 2014.
Once you accept a class date, you are expected to be there. Keep in mind, delaying class dates pushes you down in seniority. Seniority is king!!! A number is a number and it does matter.