People who work at doctors office how often they need to get a tb test?

That's not medical: States and perhaps municipalities make those regulations. You can either call or find their website for that information.

TB: We get annually at the clinic i work at (unless the person is a PPD converter).

Once per year: Night sweats, weight loss, with known exposure to active TB case. That's a different matter.

Probably varies...: The time between TB tests probably varies depending on the desires of the employer and the local laws. Just to be on the safe side, a worker who is healthy and had a negative PPD can ask for routine PPD screening every 1-2 years. A worker can also wear a mask around any coughing patients, to avoid catching whatever germs the patients have.