The stock is initially granted in the form of Restricted Stock Units (RSUs). RSUs vest according to a schedule. It looks roughly like this:

5% the first year

15% the second year

20% every six months thereafter

Compensation at Amazon typically is a combination of a base salary, a signing bonus (distributed over time), and a stock grant. The package is structured in such a way that the employee's total pay stays consistent over time, even though the pay comes in different forms. New hires will have their signing bonus structured over the first two years, and then it ends. At that point, the more significant stock vests begin to occur at six month intervals, so the total pay amount stays relatively the same after the bonus ends.

Microsoft annual bonuses used to be a lot more stock and less cash, but in 2011 they shifted towards the current model of cash++, because the stock was low and nobody wanted it. (At the same time they did an across-the-board 10% raise - from memory, both these moves were made because Amazon Google Facebook were becoming much more serious competitors for Seattle employees. http://minimsft.blogspot.com/2011/04/microsofts-new-review-a...)