If you're not already familiar with Bluetooth Low-Energy (BLE) advertisements in general, please read the Argenox primer.
The default way of getting data from RuuviTags is to listen to Bluetooth Low-Energy advertisement packets. The data is in manufacturer specific format.
Additionally, connectable Ruuvi devices have a scan responce which contains device name "Ruuvi XXXX" and UUID of the Nordic UART Service. However if you're looking to only read the advertised data, you can ignore the scan responses.
The data is broadcast and not acknowledged by the listeners, therefore there is no guarantee that any single data packet will be heard by a nearby phone or gateway. The chance to receive the data depends on the signal strength and noise on the 2.4 GHz band, typically 25 ... 90 % of advertisements can be received in good conditions.
The delay between updates is random, a gateway may miss 10 advertisements in a row even in good conditions. This latency is dependend on the broadcast interval, faster broadcasting means more chances to catch the data. On the other hand faster broadcasting will drain the battery faster, so there is a performance / battery runtime tradeoff.
As of Ruuvi Firmware version 3.28.13 the default advertisement rate is 1285 ms + random delay of 0 ... 10 ms per advertisement. Test versions of firmware advertise at a rate of 211 ms + 0 ... 10 ms and long life versions at a rate of 8995 ms + 0 ... 10 ms. Default and test intervals match Apple guidelines for bluetooth accessories, long life version might not be discovered by Apple devices at all. Android doesn't similar guidelines.
The advertisement begins with mandatory flags highlighted in blue. The the actual manufacturer specific data begins with a header highlighted in dark green which contains: