I don't know that this is a standard time period.. Honestly, the more coffee you drink, the less you are going to get the 'buzz' going. A lot of this depends on your body type, activity level, and genes.
You have to think of caffeine like a drug, because that is what it essentially is. When you take your first sip of coffee, it takes about 45 minutes for 99% of the caffeine to be absorbed by your system. The caffeine has a half life of roughly 4-6 hours in the human body. That buzz you are feeling is because as you have slept, all of the caffeine in your system has been depleted and thus when you get that first cup in the morning, your system gets a big blast of caffeine after not having any.
Here's a pretty interesting article that contains a lot of the information I just parroted above (plus quite a bit more):