The employee life cycle depends on the stage the employee is , i.e. early , mid , senior.
An average of 18-24 months for early level, 3-6 years for mid level and more than that for senior level can be broad based. however, thats essentially a generalisation and not a ground rule.
The motivation of an employee is the only measure.
Create bands and moderate on the basis of how long would the employee remain motivated at work, prewiring the market features including business risks .
You might consider reading this The right way to grant equity <link updated to site home> ( Search On Cite | Search On Google ) . I found it exceptionally insightful
Wish you all the best !
24th December 2013 From India, Mumbai