I can't find in the Ruby documentation anything on how to set a timeout to retrieve the IP of a domain using the class method getaddress from the Resolv class—from the Ruby std library.
Looking at the source code for Resolv, at line 353, I can see that there is a method named timeouts defined in the DNS class. You should be able to use this to change the timeout.
# Sets the resolver timeouts. This may be a single positive number
# or an array of positive numbers representing timeouts in seconds.
# If an array is specified, a DNS request will retry and wait for
# each successive interval in the array until a successful response
# is received. Specifying +nil+ reverts to the default timeouts:
# [ 5, second = 5 * 2 / nameserver_count, 2 * second, 4 * second ]
#
# Example:
#
# dns.timeouts = 3
#
def timeouts=(values)
@config.timeouts = values
end