My fixed spell candles are a prepared ritually with herbs, oils and washes. Each fixed candle is its own intention that requires attention and should be treated with reverence.
7 Day Fixed Spell Candles should be burned in 7 days, which makes for 1" a day. This isn't realistic for many witches, so do as you feel fits within your lifestyle.
*** Always burn candles to an even melting pool before extinguishing, or you will end up with an uneven burn and a "tunnel" like candle. An even melt pool if when the wax is melted evenly across the top of the candle from wall to wall.