I am a nanny, my jobs have been 2 years, 2 years, 2 years, 9 months (temp position for 3 months, but got extended), 4 1/2 years, 3 years, 2 years, 5 years and my current job which I have been in for 7 months and they want me to stay for at least 5 years! (We'll see)
I think it depends on the position, the parents, the nanny and how they all get on, trust me, it is very rarely the kids!!