I don't know how the warning system is worked (fortuantly I've yet to find out the hard way - only one edit to date and that was for stupid quoting), but I'd have thought that a good way to do it would be to store the date any warnings were given. That way, should I have hypothetically upset an admin and get a warning a year ago, and then get another warning today, the admin will be able to see that the one before was a year ago. Then again, if I suddenly went on a crusade against the admins and got a whole bunch of warnings, they would see that the warnings were all given out within a short time and have a better idea of what's happened.

Hope that makes sense. Quick translation: old warnings should not dissolve, but the date should be stored.