How often does Power Nap check for updates or back up?
- Mail, Contacts, Calendar, Reminders, Notes, Photo Stream, Find My Mac, and iCloud Documents are checked every hour.
- Time Machine backups are attempted hourly until a successful backup has been completed.
- Software Updates are checked daily.
- Mac App Store downloads are checked once a week.
Source: http://support.apple.com/kb/HT5394