For what it's worth, on my site we do not have an expiration date for advisories...once advised, always advised. We keep a detailed accounting of when, why and by whom for such informal advisories too. However, each rule is treated separately and users with less than 5 posts are treated more leniently with just a reminder in the thread where the rule was broken and no record kept of that. Sometimes it is a good thing for reminders to be posted publicly as other users will see that and be reminded that moderation is active and fair.
We also employ a system whereby users are required to accept the rules before they are allowed to post. Then a record is made of when they accepted the rules, and shown in their profiles for senior staff (admins/global mods). This way a user cannot say they had no idea of a particular rule and that they are being treated unfairly.
We found that users respond much more favorably to an informal PM than to getting a formal warning or infraction the first time the violate a rule. Even when requiring a user to accept a set of rules, it should be kept in mind that most will read them as thoroughly as they would read the instruction manual for a new device. Most people just want to power up and go, which is understandable.
I wish to point out that the system being used here is even more forgiving than the one I put into place at my site (where we have more rules governing how to properly ask for math help), and we have had no issues, so I think CH will be fine as well.