The topic of structural safety is an extremely important subject and is studied mainly with reference to structures and infrastructures made with materials other than masonry. Masonry is a composite material made of a variety of materials - including brick, stone, and mortar - with dry joints or mortar joints. This results in complex mechanical behaviour, featured primarily by low tensile strength and good compressive strength, and the presence of finite friction and cohesion at the interface joints. Given the above, masonry has inherent uncertainties in its mechanical characterization. To these uncertainties must be added those related to the evolution of degradation and natural aging of the component materials. These aspects complicate the evaluation of the structural response in terms of safety, reliability or robustness in historic masonry buildings. This paper presents the first step for the safety assessment of a masonry building subjected to seismic action, considering a certain level of damage through a significant performance parameter. This parameter is able to describe the reduction of the characteristic strength values due to the presence of a crack pattern induced by live-loads changing and its evolution over time. The evolution of the damage affects the global safety factor, SFG, significant parameter in the evaluation of the safety of a building in a seismic zone. In this paper, the assessment of time required to reach a SFG level of possible collapse is approached for the first time in a probabilistic way.