Ideally, we recommend trimming your candle wick before each use to 1cm. The wick should always look short and upright for a clean burn. If you have a wick trimmer then great (you obviously know your candles!) but if not, scissors should do the job as wicks become more brittle when they burn down.
Do not relight your candle when the wax level is less than 1cm as your container may get very hot without enough wax/ wick to burn properly.