There are two component monitor modes in CPF: the default 'learn mode' and 'on mode'. How long should one wait before switching to 'on mode'?

You can choose when is best for you. Learn mode, when a popup is displayed, applies what action you give (i.e. allow / block / ask) to all components of an application automatically from one popup. When this is ON, you have to allow / block each component seperately.
So, you can switch to 'on mode' after a few weeks when most of your applications have a rule set for them.