A candle is not meant to be burned until the vessel is completely empty. The safest stopping point is the one specified by the candle's warning and care instructions.
Why does wax need to remain?
As the flame gets closer to the bottom of a container, heat is concentrated nearer the vessel base and the surface underneath it. Leaving the required amount of wax helps you avoid pushing the candle beyond its intended use.
Stop sooner if something looks wrong
Extinguish the candle if the vessel is cracked or damaged, the flame becomes unusually large, the candle persistently smokes, debris enters the wax, or another condition conflicts with the warning label.
Do not try to “use every drop”
Getting maximum value from a candle does not mean burning it beyond its safety guidance. A small amount of remaining wax is part of normal container-candle use.
Always follow the warning label on the specific candle in front of you. For more care guidance, read 9 Candle Mistakes to Avoid.