The whole system seems dodgy to me. I'm assuming you're talking about Experian. I've got a few that seem to have "got stuck" from time to time. In particular I have two Barclaycards where the last update is shown as 04/01/09. (Funny that!) One is an active account, another is one closed a few days later.
When they finally get around to updating it, it will probably be retrospective... ie maybe mid-March the "last update" will show as 04/03/09 or some such. I'm wondering if the early Feb update got lost because it was around the time of heavy snow. Maybe some card companies didn't run off the data.
I have 18 accounts showing. It seems odd to me that different accounts often seem to show the same update date.
I get the same thing with searches. I recently got accepted for a new card (letter in post). Then a couple of days later, my Experian account showed the search (back dated to when it was). Then about a week later I got the "alert" email. When I check it out, it's because they review the account weekly and had "noticed" the search during the previous week.
So I can't really answer your question. Based on my 6 months experience of Experian, it always updates within 3 months (usually faster) though the "as at" date is usually a little before the date they actually update your record.
Since this data is so critical to people when they apply for credit, I think the CC companies/Experian should take more care to get it right. I feel in your case, its unlikely to make much difference if a single missed payment was the last one or a slightly earlier one. But the balance drop is more significant.