Mine is at 550, which I usually have no trouble hitting at least 5 out of 7 days in a week. Sometimes I hit it only 3 or 4 days a week though. I think you want to set it to be a little bit of a stretch, but still achievable. Of course it depends on what your goals are. If you want to be seriously fit you will want to aim much higher. If you are looking to just maintain your current health and weight, a lower number might suffice.
The watch will recommend a number based on your activity pattern, but I have kept mine at 550 even when it has suggested I set it lower or higher. That makes it easier for me to compare month to month. I'm more active in the warmer months, so there are times in Summer when I'm biking a lot and double my Move goal a couple of times a week. Then there are times in colder/wetter months when I might have to work hard to reach my goal. I'm not saying I would never alter my goal. There are times when life's circumstances make more modest goals a stretch, and there are times when we are able to devote more of our day to exercise and we can set more aggressive goals.