How are the currencies updated?
Whenever somebody calls a page where exchange rates should be displayed (that's the single property view basically) an internal function is called.
This checks the database if the exchange rate has already been update on the same day.
If not, a query is done to the echange rate server, the rate is updated in the database and displayed to the user.
This is done with all defined rates.
Caution
This process will slow down the process of building the page.
It will do that only once a day, don't worry, but in order for your visitor who is triggering the update not to wait too long you should not enter too many currencies.