AVAYA recomends infinite, or 30 days (30 days is really 15 days, because the DHCP Client starts trying to renew its lease at 1/2 way through the lease time)
I would say 30 days or infinite is fine, you don't want to cause uneeded traffic on the LAN, for the IP phones renewing their leases all the time. Most phone environments don't change that often, once they are setup.
mitch
AVAYA Certified Specialist