The idea sounds really bad. This is called
pull technology or polling, very inefficient and non-responsive at the same time. You really need to get to making proper architecture. Apparently, if XML has changed, something triggers it. This event should be used to notify the WCF service. This way, the service would play its natural role and won't do unnatural polling.
See also:
http://en.wikipedia.org/wiki/Pull_technology[
^],
http://en.wikipedia.org/wiki/Push_technology[
^].
See also my past answer:
Application 'dashboard' for website accounts[
^].
If you provide adequate level of detail, perhaps I could provide mode detailed advice.
—SA