The　user　visual　attention　has　been　playing　an　imperative　role　in　visual　Quality　of　Experience　(QoE)　modeling.　In　recent　years,　researchers　have　developed　numerous　visual　attention　models　for　images　and　videos.　However,　existing　studies　on　visual　attention　generally　perform　on　spatial　domain　(i.e.　pictures)　without　consideration　on　time-variant　user　attention.　Besides,　the　user　attention　on　immersive　video　playback　has　not　been　well　investigated　in　Virtual　Reality　(VR)　environment.　In　this　paper,　we　design　subjective　experiments　to　acquire　the　user　attention　during　360-degree　video　playback,　in　order　to　characterize　the　visual　attention　changes　on　time　axis.　We　compare　and　model　the　temporal　attentions　in　two　scenarios:　360-degree　video　and　conventional　2D　video　display.　It　is　observed　in　video　display,　the　user　attention　needs　a　short　period　(　30　seconds)　to　concentrate　to　a　relatively　stable　level.　Within　this　period,　the　user　attention　to　360-degree　video　is　significantly　lower　than　that　to　traditional　2D　videos,　which　shows　that　the　users　are　relatively　difficult　to　adapt　to　VR　display.　Therefore,　short　videos　less　than　30　seconds,　especially　short　360-degree　videos,　are　not　preferable　in　subjective　video　quality　assessment,　although　they　have　been　tested　in　some　scenarios.　We　hope　this　conclusion　would　conduce　to　QoE　modeling　during　video　display　of　VR　environment.　©　2018　IEEE.