While achievements never expire, bonuses are timed in order to help keep you on your career path and reward you for your effort.
Achievements can be thought of as Milestone Accomplishments in your career. These milestones are split up in to 20 different levels that spread across your career. The the typical career trajectory and roadmap for accomplishments are part of the businesses initial setup and can be evolved over time to optimize changes in market and business structure.
So, let's hypothetically say that the length of a particular career position equals 5 years. With 20 Milestone Achievements, they would be split evenly or 1 level to be accomplished every 3 months. These achievements are cumulative and customizable to represent a curve of your growth. Longer career paths would have the time between milestones adjust accordingly.
While you receive Coins, ZBucks and XP for the achievement, You also have the ability to push ahead of your curve and receive a bonus reward for your efforts. This can help you gain a little extra time towards your next goal(s). Good luck! It's Game Time!