Im making chicken stew in a slow cooker but when should i add cabbage?

Depends if you are cooking it on low or high. If you are cooking it on low then maybe add it in a few hours before the stew is done. If you are cooking it on high then add it one hour before the stew is done. Don't add it in at the beginning because the cabbage will overcook and turn gray in your stew and will taste very bitter unless you like overcooked cabbage.