To answer this question, we need to cover the way that Signagelive Media Players make requests to do Content and Health Checks and as part of this, we can also answer the query about Screenshots.

Outside of the Player downloading the unique media that is required as part of your scheduling, our Player API players do HEAD requests to check for new content at the configured content check frequency you have set. This check (Content Check) just returns the Last Modified header and in total, the response is around 255 Bytes.
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If a configuration update is required then the HEAD request will be followed by a GET request. The size of the response of this request depends entirely on the complexity of the scheduling configuration for that player. This could be anything from a few KB to 100s of KBs. Once the player has downloaded the updated configuration it will proceed to download any new media files that it does not already have stored locally. The amount of data transferred at this stage is entirely dependent on the size of the new media files published to the player. A Health Check, the device communicating back to our server to confirm being online, is around 1KB.

So in short, Health and Content Checks plus Screenshots (if enabled) will transact data of these sizes and then the volume will be determined on how frequently you have these checks set. Health and Content Checks can be every minute, and Screenshots every 5 minutes - however, we'd recommend setting these values higher as you become more familiar with the platform.