1. The router will keep NAT entries in the translation table for a configurable length of time. For TCP connections, the default timeout period is 86,400 seconds, or 24 hours. Because UDP is not connection based, the default timeout period is much shorteronly 300 seconds, or 5 minutes. The router will remove translation table entries for DNS queries after only 60 seconds.You can adjust these parameters using the ip nat translation command. see the following link for details.
  2. reference:  https://flylib.com/books/en/2.286.1/adjusting_nat_timers.html