Do effects of caffeine on human body change with habitual use?
I don't have any references off hand (though if you like, I will find them). The body acclimates to caffeine intake, becoming desensitized to chronic caffeine use, analogous to chronic narcotic use. Habitual coffee drinkers, soda drinkers, and so forth must achieve the same level of daily caffeine intake to maintain a normal state of being. Taking in less caffeine leads to withdrawal symptoms, and taking in more caffeine leads to the familiar stimulatory effects. As for time frame of addiction, this likely varies, but I hazard a guess it's on the order of a few days to a week. Fortunately, caffeine is not nearly as strong and addictive as narcotics, however it is the most widespread.

Edit: Caffeine exerts high-affinity and low-specificity for adenosine receptors. This was a review article I fould that may help you out. 


Dunwiddie TV, Masino SA. The role and regulation of adenosine in the central nervous system. Annu Rev Neurosci. 2001;24:31-55.