When your candle is burnt down to the silicone base, indicated by a line, it's time to pop your wax out and refill. Any leftover wax can be easily popped out and shouldn't stick to the sides.
If there is any soot or dust you can rinse it out in the sink. You could even throw the candle jar in the dishwasher, we just would recommend the jar is completely rid of any wax before doing so.