This turns out to be a difficult question to answer.  The main reason is 
that diamonds do not themselves contain anything useful for radioactive 
dating.  They are almost pure carbon, and are much too old for radiocarbon 
dating.  An internet search I did revealed some new work, published in 
1998, (http://earth.leeds.ac.uk/ygs/programme/year1998/mar98.htm#Pearson) 
that suggests that at least one diamond from Siberia may have grown over a 
period of more than 300 million years.  This work was done by dating 
inclusions of sulfide minerals (little crystals trapped inside a bigger 
crystal) within the diamond using Renium and Osmium.

Diamonds grow more than 150 kilometers deep in the mantle and their growth 
rate may be highly variable depending on the local abundance of carbon 
(which is very scarce in the mantle in general), the temperature, the 
presence or absence of melt, and other factors.

The time it takes to make a diamond should not be confused with the 
diamond's age.  A diamond could form and then just sit around for a long 
time.  Many of the diamonds we find at the surface of the earth or in mines 
are very old, perhaps 3 billion years (the Siberian diamond inclusions 
mentioned above are apparently 3.4 to 3.1 billion years old).

David Smith, Ph.D.
Geology and Environmental Science
La Salle University, Philadelphia, PA