Very, and often. The services that collate the data and provide the rates were orginally developed by a team of researchers doing deep learning analysis of various financial instruments and indicators.
They found that the existing API's were either not updated fast enough or did not consider all of the available sources that reflect the true - exact- exchange rate. Although the margin of error available from other sources was acceptable for most scenarios, the high frequency pattern analysis that was being researched required a much higher degree of accuracy.
The rates are refreshed as soon as new and relevant data is received, there is no fixed refresh time, however most of the rates change around every 15 seconds.
Yes, we know this makes us look evil, but it does keep it free! :)
Unfortunately we cannot currently accommodate very high frequency polling of the end points from hundreds of thousands of sources. We use the imagelink as a sort of payment-in-kind to determine which users should have priority without resorting to charging for the service,
We are happy for the API to be used for other systems too, and are also interested in collaborating with other research groups. Please contact us at firstname.lastname@example.org for more information.