Radio-frequency (RF) retroreflector attack (RFRA) is an active electromagnetic side-channel attack that aims to leak the target's internal signals by irradiating the targeted device with a radio wave, where an attacker has embedded a malicious circuit (RF retroreflector) in the device in advance. As the retroreflector consists of small and cheap electrical elements such as a field-effect transistor (FET) chip and a wire that can work as a dipole antenna, the reflector can be embedded into various kinds of electric devices that carry unencrypted, sensitive information; e.g., keyboard, display monitor, microphone, speaker, USB, and so on. Only a few studies have addressed the basic mechanism of RFRA and demonstrated the success of the attack. The conditions for a successful attack have not been adequately explored before, and therefore, assessing the feasibility of the attack remains an open issue. In the present study, we aim to investigate empirically the conditions for a successful RFRA through field experiments. Understanding attack limitations should help to develop effective countermeasures against it. In particular, with regard to the conditions for a successful attack, we studied the distance between the attacker and the target, and the target signal frequencies. Through the extensive experiments using off-the-shelf hardware including software-defined radio (SDR) equipment, we revealed that the required conditions for a successful attack are (1) up to a 10-Mbps of target signal and (2) up to a distance of 10 meters. These results demonstrate the importance of the RFRA threat in the real world.