When are Android apps auto-updated?

In the Play Store, I have checked the box "Auto-update" for my application. However, although the app received a new version several hours ago, it is not being auto-updated on my phone. What I'm wondering is how often are apps auto-updated (if at all)? **Edit:** It's been one day and the app hasn't been auto-updated yet. I've tried letting the mobile charge to 100% overnight but it didn't help.