Usually the Employee Share agreement has a clause that states that when you leave the company (or even in change of control scenarios) any vested stock options must be exercised by 90 days, otherwise their forfeit back to the company.
This is important. The 90 day time period is a typical clause in your options agreement. It is not universal. You should check for this and other "standard" clauses in your options agreement to ensure that your company isn't sneaking something past you when you're signing on.