Does temperature, humidity, altitude, etc. On each successive visit you can narrow down your list to some final considerations. Now, you are prepared to spend longer visits in the two or three cities you've narrowed-down to. It's time to get serious, and do a gut check to see if you really like it. And then, it's decision time.
Nothing is hard and fast.
If you locate somewhere and find you don't like it, you can always move. But, I have found that it takes me at least 6 months to calm down and assimilate to the new town. Remember, it's going to be like crawling like a baby, once again. You will be entering a culture completely different that the one you are leaving. Take things easy, accept what's out there, and adapt to the lifestyle in your new location.