carbon usually only lasts for about a week. it really isn't necessary in reef tanks and lower quality ones may even release phosphates into your water. What I would recommend is a higher quality organic waste remover, something that mimics what carbon does but is much more effective and safe. Some options would be chemi-pure or chemi-pure elite (the latter of the two has extra media in it to remove phosphates), purigen by seachem, biochem zorb, or any other products similar to these. The will last a lot longer than the carbon and some of them are even rechargable making them last even longer. Personally I love chemi-pure elite, but many people have arguments for what they use to.
I stick with carbon over the other products. It won't leach appreciable amounts of phosphate, and it will bind various organics. The organics can make the water yellow, and might be allelopathic toxins.