By default, in sampled mode the HR sampling is turned on every 5 minutes (hr_sampling_interval = 300). This can be changed via the hr_sampling_interval setting in the operational parameters characteristic. HR is measured until a reliable HR is found and the measurement is turned off. This duration varies but during daily life it is on average 12 seconds. During those 12 seconds the HR is stored to the device internal memory based on the hr_data_interval setting in the operational parameters characteristics, which is 1000 ms by default. 

Note that the hr_data_interval only affects the frequency at which the HR is stored to the device filesystem, streamed to the phone over the BLE Object Transfer Service and stored to the database in the /sdcard/PulseOnOHR/ directory. The setting doesn’t affect the frequency of the real-time HR value that is sent over the standard HR service or over the HR data characteristic of the Device Control Service. The HR is sent every second over those.