This paper presents a Monte Carlo-based framework for the estimation of lifetime reliability of multicore systems. Existing mathematical tools either consider only the time to the first failure, or are limited by their intrinsic complexity and high computational time. The proposed framework allows to compute quasi-exact results with a reasonable computational time, without adopting typical (and possibly misleading) simplifications that characterize the existing tools for computing Mean Time To Failure (MTTF). The paper describes the framework with all its mathematical details, assumptions and simplifications; it proves the correctness of the obtained results, by comparing them against the exact ones, and underlines the differences with the simplistic approaches, also discussing time overhead improvements.