FWIW, I've had three in my 25 or so hours - one for the trial lesson, a second who left for the airlines after three lessons and the third one ever since. Unless the current one also gets an airline job then that will be it...
Have you gone solo yet? If not, then I'm not sure that constantly changing is a good thing - it seems to me that you really need an instructor to have a good idea of your progress at that point - but apart from that I cannot think of a time when it would really cause problems. Apart from circuit work so many of the lessons are self-contained that I doubt that it really matters much - as long as you're happy with it.