The first three months are essential because you need to make an impression during this time, meaning that you should be extra attentive and work hard to exceed the expectations of your boss and colleagues. In addition, it's an excellent time to establish some work-life boundaries. For example, if your boss is pushy about working on the weekends, don't hesitate to set some clear limits. If you can accomplish all of this in the first 90 days, then you're on the right track to success.