K. M.'s post elaborating on TAN issues is correct. You can use the suggested rule of thumb (for setting your TAN limits for industrial oils). However, my advice is to at least yearly run ASTM D 2112, Oxidation Stability Test. Then you would see clear pattern; antioxidant reserve dropping and TAN slowly climbing. It is not advisable to continue to use oil which antioxidant's reserve depleted more than 80% from its original value, because at any point after that, TAN number would start rising more rapidly.
With automotive lubricants, TBN depreciation is the parameter to check and track, and replace the oil when before it approaches zero. Consequently, you are not supposed to see TAN number in those oils, as acid would start being very active (corroding) at those high operating temperatures