.... For MSDN Keys:- MS can only act once a key hits a threshold (and they won't tell an outsider what that threshold is, understandably). They then have to put it through internal systems to ensure that all relevant parties are informed, just in case it's an oversight - then it enters the queue for the next update to the system.
I have no idea how often the WAT checking system is updated, but I would suspect that it's no more than weekly because of admin and timing issues. The internal MS processes could therefore take easily a couple of months. It therefore depends on how close to the end of a vendors sales list you are, and how quickly he's selling them.
The cannier ones will sell just enough to stay under the threshold for each product (MSDN subs include a huge number of products), until they've maximised the profits, then try and flood the market before disappearing into the sunset, laden with your cash.
This could take anything up to a year or more.
