Your hair is vulnerable to chemical treatment, and coloring is such a process. As such, you should only re-color your hair once every 6 weeks, and not more frequently as it will damage the fine structure of your hair.
 
If you want your color to last longer without much fading, use shampoos and conditioners that are designed for color treated hair. Deep conditioning treatment will keep the color longer. Look for products with a hint of color in them; it will help your color to stay and stretched the interval where you need to color your hair again.

People with short hair may reduce the interval of re-coloring as you trim off the ends regularly. However, if you have long hair, be careful of over-treatment due to frequent coloring.

Of course, how often you can color your hair will also depend on how fast your hair grows. You may wish to know that hair grows faster during summer, so you can re-color more often. However, to keep things simple, just follow what qualified hair professionals will tell you: that is, as a general rule of thumb, 6 weeks is a fairly reasonable interval between each treatment.