In recent years, monitoring has offered a viable complement or even alternative to traditional analytical safety verification approaches. However, there still remains a lack of guidance for the use of monitored data in safety verification work. Limited resources often necessitate relatively short time frames for safety verification of problematic bridges. While the duration of monitoring is always an important consideration, it is rarely examined explicitly in terms of its influence on the predicted characteristic action effects. This work gives a novel approach for determining if a monitoring regime is sufficiently long to obtain a reliable extreme value estimate of the action effect. A case study of a highway bridge in Switzerland, with one year of continuous high-frequency measurements, is used to exemplify the approaches developed. The focus is on direct measurement of the traffic action effects in the bridge's deck slab reinforcement. A comprehensive statistical approach for determination of site-specific, element-specific extreme traffic action effects is presented.