The paper deals with the problem of estimating the execution time of firmware. Any firmware is bound to wait for a response from peripheral devices such as external memory chips, displays, analog-to-digital converters, etc. The firmware’s execution is frozen until the expected response is obtained. Thus, any firmware’s execution time depends not only on the computational resources of the embedded system being inspected but also on peripheral devices each of which is able to perform a set of operations during some random time period residing, however, within a known interval. The paper introduces a model of a computer application for evaluation of microcontroller-based embedded systems’ firmware’s execution time that takes into consideration the type of the microcontroller, the total duration of all the assembler-like instructions for a specific microcontroller, all the occasions of waiting for a response from hardware components, and the possible time periods for all the responses being waited for. Besides, we proposed the architecture of the computer application that assumes a reusable database retaining data on microcontrollers’ instructions.