It depends. If you just got a really bad hair cut, then hair seems like it does not grow at all, while hair that you have to shave every day seems to grow like a weed!

Ignoring these perceptual differences, human hair seems to grow at a pretty consistent rate of about one half of millimeter or so per day, or about half an inch every month (This article quotes a specific rate of 0.44 mm per day). It may grow faster or slower depending on your age, your genetics and your hormonal state (pregnancy seems to have an affect on hair), but one half of a millimeter a day is a good average.

What this means is that the maximum growth rate for hair is about 6 inches (15 cm) per year. If you have short hair and are thinking, "it sure would be nice if I had hair down to my waist", then you are going to have a four-year wait!

Here are an interesting link that contains a wealth of information about hair: The growth and loss of hair.