I have been running the campaign performance report with a date range of current month and and reporting by day.
It appears my stats for the various campaigns I am running are being updated to the previous day. Of course, I cannot confirm this, however, I have been monitoring over the last 10 days or so.
Stats (clicks, costs, etc.) have not changed (increased) for dates older than 2 days. So, for example, today (5/30), my stats for 5/28, 5/27, 5/26, 5/25 were the same as the reports I pulled yesterday. This is telling me (hopefully) that these stats are updated and accurate.
Assuming this, I simply update 1 day late (tommorrow, I will assume my 5/29 stats are updated).
This has been consistent for all my campaigns for the last 10 days or so.
Of course, I have no verification these are updated and final stats (hopefully MSN won't drop a big "dump" of clicks/costs on day prior). But, this is about all I have to go by.