Depends on the moisture content and storage conditions. Depending on how the tar was made (acetic acid vs acetic anhydride/acetyl chloride) there can be differing amounts of water in it to start with. It should be stored in a sealed container in a cool dry place if possible. If it's wet the shelf life can be measured in days before it turns to morphine (or starts growing mold). Otherwise if it's dry it will store for a long time.