Diffie Hellman key pair lifetime

From: Jonathan Browns (jonathan_browns_at_yahoo.com)
Date: 11/07/03 

Date: 7 Nov 2003 04:45:34 -0800

Dear sci.crypt,

I am using the Diffie Hellman key exchange algorithm on an old machine
resulting in slow times on key generation.

My question is: How much can I reuse and for how much time a
Public/Private key pair in Diffie Hellman?

I read on the Applied Cryptography by Schneier on the "Key exchange
Without Exchanging Keys" section (page 515 on 2nd edition) that "The
public keys have to be certified to prevent spoofing attacks and
should be changed regularly", but I don't have an idea of what it
means precisely "regularly".

On the RSA FAQ Section "What is Diffie-Hellman?" (
http://www.rsasecurity.com/rsalabs/faq/3-6-1.html ) at the final
paragraph I read:
"The parties' key pairs may be generated anew at each run of the
protocol, as in the original Diffie-Hellman protocol."
But it says: "may" and doesn't explicit what happens otherwise.

Thank You Very Much,
Jonathan Browns