I've got a 2851 running 12.4(11)XW5 where I run CallManager Express, some IP accounting and NAT.
Unfortunately while it all works painlessly, SSH sessions will timeout after idle inactivity going out of the NAT interface to the outside world. The reverse is also true, where SSH sessions to a server behind the NAT interface via a NAT static translation rule will also suffer the same timeout.
NAT sessions internally are fine, as are sessions externally via my IPv6 tunnel. These servers are fine as my sessions stay active for days from other systems.
ip nat translation timeout never
ip nat translation tcp-timeout never
but with no success.
sh ip nat stat:
Total active translations: 120 (1 static, 119 dynamic; 120 extended)
GigabitEthernet1/0.1, GigabitEthernet1/0.120, Loopback0
Hits: 192029370 Misses: 4998710
CEF Translated packets: 179702310, CEF Punted packets: 35974282
Expired translations: 6830497
-- Inside Source
[Id: 1] access-list 102 interface GigabitEthernet0/0 refcount 31
Queued Packets: 0