Web-service-related techniques have become popular to improve system integration and interaction. In distributed and dynamic environment, web services' availability has been regarded as one of the key properties for (critical) service-oriented applications. Quality of Service (QoS), including availability, has been regarded by IEEE as a userperceived property. However, based on our investigation of monitoring invocation records of real web services, existing availability metrics, which were proposed in traditional domains, have not addressed the "user-perceived" characteristics. Based on analyzing the limitations of the existing availability metrics, we propose a status-based userperceived service availability metric and a corresponding estimation approach. Experiments on monitoring and analyzing the invocation records of real services demonstrate that the new metric and the corresponding estimation approach could lead to a feasible estimation on web services' availability from the user side.