A candle that tunnels — burning a narrow well down the middle and leaving a thick ring of wax stuck to the glass — has usually been ruined in its first hour, not its tenth. Wax has a memory. The width of the pool on your first burn is the width it will try to return to every time afterwards.
Give the first burn three hours
Light the candle when you have three uninterrupted hours ahead of you, not fifteen minutes before bed. The pool of melted wax needs to reach the edge of the glass all the way around. Once it does, you can put it out. From then on, the candle will burn edge to edge without being asked.
If you blow it out early on that first evening, you will spend the rest of the candle's life chasing a tunnel that will not close.
Trim to 5 mm, every time
An untrimmed wick does three things, all bad: it produces soot, it mushrooms into a black bulb, and it burns hot enough to melt through wax faster than the fragrance can carry. Five millimetres is the number. Nail scissors work; a wick trimmer is nicer but not necessary.
Four hours is the ceiling
Past four hours the glass gets hot enough that the wax starts to burn off rather than evaporate as scent, and the wick begins to lean. Put it out, let it cool, light it again later. You lose nothing.
How to put it out
Blowing sends a plume of smoke through the room and can push the wick off-centre. A snuffer, or pressing the wick briefly into the melted wax with a metal tool and straightening it again, ends the burn without the smell of an extinguished candle following you around.
Where you put it matters
Draughts — an open window, a doorway, a heat pump — pull the flame sideways and make one wall of the glass do all the melting. A still corner of the room is worth more than any candle-care advice on this list.
The last hour
When about a centimetre of wax remains, stop. Burning past that point risks the heat reaching the base of the glass. Pour hot water into the vessel, let the wax lift and set, and the glass becomes a holder for tea lights, hair pins, or nothing at all.
Done properly, a 220 g candle gives you about forty-five hours. Done badly, twenty. The difference is one evening of patience at the beginning.