A general rule of thumb for determining when a candle needs to be replaced is when there is about 1/4 inch of wax remaining at the bottom of the container. At this point, it's best to discontinue use of the candle to prevent overheating of the container. Additionally, you can use the "burn test" which is lighting the candle for 1 hour for every inch of the candle's diameter. If the candle burns down more than 1/4 inch, it's time to replace it.