How much radiation am i being exposed to in ap full spine and what are the indications for full spine x-ray?

It depends: The health physics society site is an excellent resource on radiation dose and effect: http://hps.Org/. The dose is dependent on several variables, most importantly the patient's size. It takes more radiation to make and x ray of an obese person than a thin person. The average dose for an ap full spine is likely in the 2.5-3.0 msv range. This is about equal to 1 year of ambient radiation.