2/3 full is about right, If thats liquid. What counts is how much solids on the bottom, which should not get over 18" or so. Without a disposal which is septic death, I go about 15 years easy and get about 12" solids build up... I think the septic guys say 5 years which might be right for sand filters on the outlet and with lotsa kids and disposals. When the odor is regular, get worried.