[Cryptography] How often should the DH group be changed?

[Pierre Abbat]

Let's say that there's a group of computers that maintain network connections 
to each other, and they send information to each other encrypted with a 
symmetric cipher whose key is agreed to with the Diffie-Hellman key exchange. 
Every n days they pick a new group at random. How often should they change the 
group for various sizes of the Diffie-Hellman group? I'd like to know both for 
the multiplicative part of a prime field and for an elliptic curve.

Pierre
-- 
When a barnacle settles down, its brain disintegrates.
Já não percebe nada, já não percebe nada.