The ability to know when it's time to move on is a skill all of it's own.
Circumstances change and it's important to make sure your radar is on so that you notice when things are happening.
Change can mean there are significant opportunities to look out for - like new jobs or projects that you want to be part of.
Or change can mean restructuring, downsizing or a new boss - any of which can require you to put your career 'Plan B' into action.
Network. Think about what you see and hear. Observe the behaviors of senior management.
Don't become preoccupied - but keep on the lookout for when there is change ahead.
Have you observed change in advance at work? And what did you do about it?