不是5年。
如果job offer letter (4/25/2011), 4800 shares of option, purchase price at grant date price
1. 4/25/2012, 1200 share to exercise
2. 5/25/2012, additional 100 shares to exercise (4800*1/48)
3. 6/25/2012, another addiotnal 100 shares to exercise
........