In wireless communication systems, the Friis transmission equation is commonly used to quickly estimate wireless transmission efficiency between the transmitter and the receiver. In the case of wireless implantable bioelectronic systems, the gain of the implantable antenna is significantly decreased by the high-loss nature of the host body. Additionally, numerical analysis of implantable antennas considering the host bodies is time-consuming and laborious. On this basis, this work proposes a method for quickly estimating the link efficiency between an implant and an external node, which is applicable for implantable miniature antennas located in a large-scale host body. Compared to the previously proposed approximation of in-body path loss, this work extends the assessment of path loss to external nodes in free space. The method uses closed-form expressions that consider various parameters, including implant dimension, implantation depth, and the permittivity of the host body. A measurement example is provided, validating the effectiveness of the proposed method in the assessment of the link budget for wireless bioelectronic systems.