I have had three normal nannies, and indeed one of them, aka LovelyNanny, was on my sofa this morning working out with me how she can help out until I find a more solid solution (she is due to give birth very soon so we are having to take it a day at a time).
I had my first AP in 1987! I have had something like 20 APs and 5 nannies over 27 years. Some APs were only ever engaged for 3-4 months over the summer, that was always the intention. Very few around here in the UK do more than 9-12 months. 6 months is very common. Some have extended their stay with us because they were enjoying themselves. Some come back to babysit sometimes, if they are still in the UK.
As MN old hands will remember, our record for nutty nannies is the one who came for one hour or so, had not actually started work yet, was having a coffee and took a call from her mother who said she was in hospital having a heart attack, DH said, "You must go to see her!" and waved her off, girl never spoke to us again, after 3 days we rang The Place We Don't Name and asked what was going on, we were told the girl had changed her mind about taking the job. NOT very professional. Eden Nannies said she had form doing this and they didn't like trying to place her as she was so unreliable, she preferred riding her pony and was a bit of a princess.
The runner up to the nutty nanny prize is known to Nom de Clavier, as this nanny's college cohort were no better than her. She refused to lock the front door or car as 'That is proxy parenting' and tore bibs off my baby when I wasn't there and made his neck have raw red lines, and refused to acknowledge the fact she was doing it. She tried to serve the kids almost raw meat on one occasion, and on other occasions just didn't feed them properly, they were hungry. She crashed the car (no kids in there at the time, luckily). She was trained at the private college We Do Not Name and had done a two year cookery/nutrition course, food hygiene course, special driving course, and studied baby care for two years. I think they may just have struck her from their register but I can't be sure (maybe Nom de Clavier can tell me as I have always wondered).
I have neighbours who have nannies and APs around here and their experiences are very similar. One of my neighbours just sacked an AP as he slapped her DS, and another AP she had stole a lot of money from her (and other people). I also had a thief once. I just think it can be as fraught a relationship as any working one, except you are often living with people as well, which exaggerates any problems.