The recent Quora question: How do you know when it’s time to leave your current company and move on? is a good post to read no matter where you are in your career and regardless of your happiness level in your current job.
As many of the respondents point out, there are red flags on both sides: your employer/boss and you.
In terms of your employer, use your observational skills. Are you being passed over for promotions, projects or new equipment? Has your daily interaction with coworkers and your supervisor become perfunctory? Have you had a recent review or are on a professional improvement plan? If you have had a review or are on a professional improvement plan, do you want to make the changes necessary? If you don’t, that’s a very clear indication that you should begin immediately looking for something new.
Regarding the you portion of the equation, are you still learning? Do you dread getting up in the morning? Do you watch the clock tick slowly toward 5 p.m. or the end of the day? Do you want to perform your job responsibilities or functions better? Is the reason you joined the company or organization still relevant to you?
If you can’t accurately assess your situation or happiness on your own, ask a trusted friend. Once you have a good idea of where you stand within the company and what your priorities are, make a decision. Then think about that for a full work week and decide if you’re happier knowing you plan to leave or plan to stay.