How long before a composite filling becomes discolored?

It depends: With today's improved composite fillings and with modern polishing techniques, these fillings really tend not to discolor, even over many years. If you are a heavy coffee or tea drinker, or smoke, it increases the chances of staining or discoloring. I have seen these fillings stain in a year and have also seen some of my fillings last 15 years without any breakdown or discoloration.

Not at all: When done under the right conditions and indications, and using the newer composite fillings, the composite should not really discolor. What really happens in these cases is that if the fillings is not well adapted or you have a poor hygiene, the tooth itself where it meets the composite, the tooth will decay and leakage under the composite will occur and it look disclored.