The answer to this varies based on your hardware (and has almost nothing to do with the app). That being said, the Elite HRV app is designed to remain connected as long as you are within signal range (even if some heart beats are dropped due to distance).
At this time most available wrist band monitors, watches, and finger sensors that use Pulse Oximetry, LED, or light-based sensors to detect heart rate do not accurately measure (or do not support) R-R intervals.
The Elite HRV app can be found on both Google Play and the Apple App Store. If your phone or tablet supports Bluetooth 4.0, and has either Android 4.3+ or iOS 7+, then it is likely compatible with Elite HRV.