There isn't any firm dates. It's based on availability only. Apple will not allow for its employees to buy something that is in demand by customers and that stock is not high enough to fill that demand. Plain and simple.
In my experience, this is not normally that long. I got my iPhone at the end of July(07) after they came out. I've bought a couple of MBPs pretty close after updates before - not as much demand.
Also something to keep in mind, is that Apple will only let their employees buy one (1) of each item in a calendar year or product revision.
Meaning is they purchase a uni-body mbp, then they won't be able to buy another one until the uni-body gets another update.
Hope that helps.
(btw if you can make friends with an Apple employee, it is highly advisable to do so - I've been buying top of the line MBPs for years now, ones that normally cost $3100 - getting them for 2400-2600 and then selling them after a year of use, and still making money on the deal. It's awesome.)