the time taken decrypt the private keys (Private Random number) sent during the Diffie-Helman process depends on the prime modulus used (mod p in our case)...the biggger the prime modulus..the longer it will take, also take into consideration the magntitude of the generator in this case "g"....here is another good link which explains the "Alice and Bob" theory..the mathematics might be a little messed up for some...
http://www.academia.edu/1958665/Eucl...urity_Protocol
GRAPHICAL PATTERNS in Exponential functions that inlcude the modulus of private random prime numbers might be a breakthorugh in the future...