schools vary, but most are 18 months to 2 years of study and internship. It is a wonderful field and has opened up into the MRI, CT and other marvelous diagnostic equipment. Good luck.
All X-Ray programs are associate degree programs, so figure on 3 years to do this. You have to meet your gen ed requirements before being accepted into the program. Once you've done that, you will spend 2 complete years (including summers) between school and the hospital you will be doing your clinical studies in. You will have an associate's degree upon completion of your studies.
You will have exposure to the different modalities as you rotate through them, but you will be eligible for your registry exam in X-Ray only. You will be an R.T. (R) with the "R" standing for radiography. In order to do other modalities, you can train formally or informally on the job, but you will be required to take a registry in each of them, as hospitals are all beginning to require this now.
The fields in X-Ray that have the most jobs out there are CT and MRI. Ultrasound is another area that is in dire need of qualified sonographers. Nuclear medicine is not as hot of an area as it once was. Mammography is another specialized area, but there aren't many jobs in that area. Angiography is slowly being replaced by CT and MRI. Cardiac catheterization is still a good area, but I don't see many jobs open in that area either.