Traditional challenges for end-to-end streaming services are harder when considering 360° video content. One of these challenges relates to the lack of standardized methodologies for subjective 360° video quality assessment, especially for streaming services. This paper presents a methodology for 360° video streaming services, which inherits a lot from the existing International Telecommunications Union standards for subjective quality evaluation of 2D/3D video but also incorporates omnidirectionality. A new metric called similarity ring metric (SRM) is introduced. It measures the degree of similarity in watching patterns of a single subject or between different subjects for several subjective assessment tests. This metric enables a rejection criteria for test results. This paper also presents visual fatigue results for a subjective quality experiment.