Life-cycle cost modeling can be used to compare expected performance of corrosion protection systems over the entire life of a steel structure. Life-cycle models consider not only initial application costs, but also future maintenance costs. Critical, however, to the value and validity of the model are the assumptions used for the longevity of various corrosion protection systems and the timing for maintenance interventions. Coating durability guidelines for corrosion protection coatings estimates the time to first maintenance and also provides a schedule for subsequent maintenance actions. Fortunately, many steel structures exist with various corrosion protection coating systems so that cost models can be verified against real-world performance. While guidelines for liquid applied organic coatings compare well with observed real-world performance, estimates for the durability of duplex zinc coatings significantly underestimate the life of the coating, thereby overestimating the life-cycle cost. Real-world performance of organic coatings and duplex zinc coatings will be compared to illustrate the validity and limitations of coating durability guidelines used for life cycle cost modeling.