It entirely depends on what you do with the data it returns. Assuming you're either polling periodically or the API has some kind of push service (I've never heard of it before, so I have no idea), you would need to store the data it returns in a reactive data source: probably a Collection or Session variable, depending on how much persistence is required. Any Meteor templates that access these structures have reactivity built in, as documented here.
Obviously, you will probably need to be polling the API at an appropriately regular interval for this set up to work though. Take a look at Meteor.setInterval, or the meteor-cron package, which is probably preferable.