Ann: December 2017 Quarterly Activities and Cashflow Report, page-99

You can certainly exercise the options anytime after they have been issued to you, and hold the shares for as long as you want. However, there are many factors that might make you want to delay purchasing the shares, or do it in a more "sophisticated/convoluted" way.
For example, you might not have enough cash to exercise the options until maybe closer to the deadline. Or, some might like to use leverage (borrow money) to exercise the shares, maybe because their cash is tied up in other assets that they don't want to liquidate yet.