Au pairs typically work morning and afternoons for 15-35 hours per week and are free in the evenings and on the weekends to enjoy their own interests and activities although these hours will be dependant on the specific family and their needs.
You can work as an au pair in nearly every country around the world although there are some popular locations such as Australia, New Zealand, England, Italy, Germany, USA, and Canada.
There are some great sites such as Au Pair World, Great Au Pair, and Au Pair.
You will need to consider the minimum and maxiumum ages of the children you are willing to care for, whether you want to live in the city or countryside, which country or countries you are willing to work in among other things.