Kentico caches a lot of data under the hood. To answer your question I'd like to know what data are you retrieving with REST service:
* in case of pages - some or all data might got cached depending on Kentico settings (Settings -> System -> Performance)
* if you're talking about custom table data - I believe it won't be cached by default
Actually it is quite easy to check - use Kentico SQL and/or Cache debug tools:
* with SQL debug you'll see if Kentico runs SQL query for second request
* with Cache debug you'll see what data get's cached
I'm not sure there is a way to configure caching for REST calls, so you might need to consider custom solution. There is an option to implement proxy API that will add any extra logic (caching in your case), but still will pass the request to Kentico REST API. See more details here.