Prevent Automatic Installation of iOS Update I know that iOS devices will automatically download the latest version of iOS when it's available. A message then appears, asking if the user wants to install the update later or now.

I have 2 questions regarding this: One, if I click "later," will I receive the message again, and how frequently would I receive this message?

And two, if I restart my iOS device, would the new version of iOS be automatically installed when it turns back on?
For iOS 7 -> 8

You do not receive the message again, but the update is available through the Settings app. 

Device restarts do not cause update installation. 

Source: my wife's iPhone 5S, which went weeks and multiple reboots without installing 8.4

For iOS 9

I believe iOS 9 will prompt you daily (I upgraded immediately, but I am basing this on the comments below). However, rebooting will still not cause the update to install.