How long to wait between normal chest ct that showed mild interstitial markings lungs & high resolution ct to see any changes? 6 mo? 1 yr? no differe

Try plain chest xray: CT scans have very high doses of radiation compared to a plain chest xray. You had some findings that could be followed with a plain chest xray. If there were significant changes on the next xray, then you may need a Chest CT. If not, I suggest talking to your doctor to postpone the CT. Because you are 31 yo, you need to be aware of the amount of radiation your body is receiving.