Well, to clear some things up for you... ACLs arent exactly 'cisco firewalls' cisco has a spacific IOS for its firewalls that can actually be loaded onto most routers.

Anyway, aside from that.

I do remember that there is a timer that can be set, i'm not at home so i dont have my manuals next to me so ill get back to you on that. but i do know that you can more then likely clear it by typing the command "clear ip nat tran *" or something of that nature, the clear commands are very helpfull.

When i go back to my house ill try to look for the timer that clears it.