Many spells require the use of a candle that will take longer to burn than you have time available to supervise. It is perfectly acceptable to cut down your candles before you use them or charge them magickally. A 12 inch taper can make four very nice spell candles. Be careful cutting them! They roll around and can slip, so you also have to trim some of the wax off one end of each of the pieces to expose the wick. Save these wax trimmings and make your own candles out of them for Yule or Imbolc.
Occasionally you will come across a spell, or develop one yourself, that requires the use of a seven or nine day candle, or even a candle that is re-lit each day for several days. Of course for these workings you will need to use a larger candle or the one specified in the spell instructions. There are also workings that will call for multi-colored candles. You can dip small white candles in colored wax (crayons can be used if necessary) to fulfill this requirement.
However, in some cases this is not best. There are rituals that work on the chakras or help you to achieve altered mind states that utilize rainbow colored candles that can take several hours to burn. These hours are sometimes necessary to achieve the desired results.
If you are adept at focusing your intent, there is no need to sit in front of a candle for 15 hours while it burns itself out when you've already done the real work. If you enjoy sitting for long periods of time staring at candles until you fry a spot in your cornea, that's okay, but generally there are other ways.
In recent years, a few companies have come out with small candles in hundreds of colors. They are about one centimeter in diameter and about 12 centimeters tall. They burn for about 1/2 to 1 1/2 hours and are great for rituals. They even have tiny candle holders of all types; from 5 pointed stars and miniature taper holders, to small ceramic cats and strawberries.
If you can't find those, you can get Hanukkah candles around (before) Yule time that are inexpensive, high quality, quick burning, and varied in color. But then, cutting down big candles works too.