But assumptions can be incorrect, incomplete, or dead wrong. Your first step as a manager must be to ensure that you understand why you were asked to manage, and to ensure that you and the company have the same expectations. Knowing is your job and your responsibility. Some companies may help you to know what is expected of you better than other companies, but the responsibility for knowing rests with you, not with the company. A manager should always understand the timeline.
Managers frequently make false assumptions about how much time they have to achieve goals.
Assumptions about time are almost always wrong, and can be very harmful to your career. Many managers assume by default that they have an infinite amount of time available to them. That is never true in business. Realize that as a manager you must show quantifiable results in a measurable timeframe. How much time you have will shape how you manage.