In general, how often does a full mouth series xrays need to be taken? once a year? every 3 years?

Varies: Depends on the dental condition of the patient, active periodontal issues, and caries susceptibility. For some patients, three years is fine. Others may require more frequent films. No one set answer for everyone.

Varies: x-rays are diagnostic tools. Depending on your gum/bone and tooth length, your Dentist may choose to take more or less frequent x-rays. Recommendation is no less often than every 3 years, but some people require more frequent visualization of tissues that can't be seen with the naked eye. Discuss frequency with your Dentist, and follow his/her recommendation.

Depends: on age and your dental issues. If you are an adult and have good oral hygiene with relatively minor issues, every 3 years can be fine. But if there are any areas of concern for your dentist, keeping track of these with annual x-rays is wise.