What is the right time to take GMAT?

GMAT scores are good for five years. So as long as your score is good by the time you enroll, you're good. I suppose schools may look at a more recent score more favorably. But a score is a score and the GMAT is just one component of your application. If you have a specific question about that, I would email the school directly ask them how they feel.

You should take the GMAT when you feel you have enough time. Yes, it's hard to balance studying and working, but it's hard to balance anything with your full time job. Also, it's hard to advise you without knowing how much you need to improve and what your target it. If your practice tests are at 500 and you want a 750, then you'll need a lot of time. But if your practice is at 680 and you just want to break 700, then you probably won't need as much time. However, I'd say that if you feel that you can fit in 8-10 hours a week of studying for about a couple months, then it's the right time to take the GMAT.