A resource estimation approach, specifically oriented towards long-lived software being actively evolved, is proposed and investigated. The approach seeks to be coherent with empirically grounded knowledge including the observation that the software progresses through a series of stages during its lifetime and that a distinctive model characterises the economics of each individual stage. Instead of a single estimation model for the whole lifetime of the software, the approach calibrates models to each stage. Its feasibility was assessed in case studies using industrial data, yielding an accuracy (in magnitude of relative error - MRE) between 20 and 33 percent.