Understanding the long-term degradation of Ti-6Al-4V under oxidative conditions is essential for studying microstructurally selective corrosion in biomedical alloys, yet quantitative, time-resolved methods to track selective phase dissolution remain limited. This study investigates the corrosion behavior of Ti-6Al-4V exposed to phosphate-buffered saline (PBS) containing 1 M H2O2 for up to 10 days, with a particular focus on quantifying the progression of β-phase dissolution and on developing an in situ electrochemical impedance spectroscopy (EIS)-based methodology to monitor dissolution depth over time. EIS, open-circuit potential (OCP) monitoring, and time-lapse microscopy were combined with equivalent circuit modeling (ECM) and finite element modeling (FEM). Immediately after H2O2 addition, the polarization resistance dropped sharply. Microscopy revealed selective dissolution of the β-phase, progressing uniformly to depths of several tens of micrometers, while the α-phase remained largely unaffected. Dissolution continued steadily at an average rate of 0.23μm/h. EIS spectra exhibited a de Levie-type impedance feature directly linked to dissolution depth, in agreement with FEM and ECM analyses. In addition, one-dimensional finite element modeling elucidated the time evolution of the resistive contributions to the EIS response. A phenomenological model was established to predict the propagation of β-phase dissolution based on EIS data, exploiting a quantitative relationship between EIS-derived relative capacitance and the exposed α-phase surface area observed by time-lapse microscopy. While the applied peroxide concentration exceeds physiological levels, the methodology presented here enables non-destructive, time-resolved monitoring of corrosion progression and is directly transferable to longer-term studies employing more realistic simulated inflammatory conditions.