- Android: Your photos are backed up as you take them (can be limited to Wi-Fi)
- Mac/PC: Your photos are backed up every hour and also when you launch the app.
- iPhone/iPad: Your photos are backed up when you launch the app (can be limited to Wi-Fi). They are also occassionaly backed up in the background, but the exact schedule is controlled by Apple.
For details on background upload on iOS, see the question: 'How do I ensure my iPhone/iPad continues to backup new photos in the background?'