When is the right time to go on a job interviews?

When you are out of work? Too late. You lose your opportunity to leverage your career and you may feel pressured to take the wrong job.
Whenever you are invited to interview? Too often. You are taking away from your current job and wasting people's time.
When the opportunity sounds like it may benefit your career? Yes. You can learn a lot about your industry, your competition and make new contacts by selecting good interviews to go on. You should be prepared by talking with a recruiter or using 1/2 hour to research the company on the Internet. Always be ready to sell your strengths and ask good questions. Go into the interview with a positive mind, interview as if you already know you want the job. After the interview, not during it, is the time to evaluate the opportunity and decide how it fits with your career plans. Whether you end up wanting the job or not you always want to leave people with a good impression. Follow up the interview with a letter expressing your enthusiasm for the position or a thanks for your time but no thanks.