You decide yourself when you work and how many days or nights you are available. We do ask that you are available to babysit for at least half a year.
Because you are in charge of deciding when you work, it is important to keep your calendar up to date, so that you don't receive booking requests for days or times that you are not available. You can block your Angel calendar for a longer period too, in case you are planning on going on vacation or for your exam period.