When we look for an au pair we prefer those who will stay for more than 3 months, with 6 months the ideal.
We have 2 children 7 and 5.
Our au pair duties are:
supervise kids watching tv after dh has done breakfast/dressing with them and gone to work. Put children onto school bus.
She then empties the dishwasher, puts in breakfast things, wipes kitchen surfaces and hoovers kitchen floor (twice a week she mops as well). Then she puts our robot hoover into one of the living rooms and sets it off. Once a week she dusts the living areas (ie. not bathrooms, loos, bedrooms)
She also walks our dog for 30 mins.
Then she is free (about 10am usually) to go out/read/surf etc until 4pm when bus drops kids back..then if I'm not home she supervises their play (they veg out in front of tv after hard work at school) and gives them a drink/biscuit..if I am back she still tends to stick around and play with them but she doesn't need to. She tends to eat with the children (I cook) at 6pm and either goes out with her friends or watches tv/surfs net in her room in the evenings.
We say we need babysitting twice a week but in reality it's more like twice a month..and we leave the children ready for bed, she just has to tell them to go to bed (they are usually watching a dvd).
Language-wise - I want to be sure that my au pair can call the emergency services and get the message across if there is any need to - so I check their language skills by phone.
Character-wise - I look for someone who mentions wanting to spend time with children in their application ( a lot of APs tell you all about how being an AP will help them, but not so much about what they can bring to us). I like it when APs have a good reason to come ie. need to improve english for a uni exam, or need to be fluent because they work in hotel industry. We also prefer APs who have thought ahead and sorted out references rather than giving me phone numbers (no email) that have to be rung at certain times. We are ok with written references (which we translate), and an email address means asking for clarification is quite easy.
Discipline etc - we give the AP a handbook which includes instructions on how to discipline our children ie. ask once, ask a second time, count to 3, (at this point they usually do as asked), if they still refuse then tell them to sit on the bottom step for a minute. I always back up the AP if I'm at home and she tells the children off for something (even if I feel she is being a little harsh) as I'm well aware that she needs to be respected by the children if I am not at home to back her up.
Look for a family who will give you a good job description so you don't end up working 60 hours etc..if they have a current AP then ask to speak to them (I always offer the email address of our current AP to any prospective APs who have got through most of our vetting system since I think she will let them know what it's like here from the AP point of view, and I value her judgement on whether she thinks they are ok or not (in case any 'madness' is more obvious to a native language speaker!)
Good luck. Hope you find a great host family.