Sunan An-Nasai, Chapter: The Book of Salah

"We used to estimate how long the Messenger of Allah (saw) stood when praying in Zuhr and Asr. We estimated that he stood in Zuhr for as long as it takes to recite thirty verses, as long as Surat As-Sajdah in the first two Rakahs, and half that in the last two. And we estimated that he stood for as long in the first two Rakahs of Asr as he stood in the last two Rakahs of Zuhr, and we estimated that he stood half as long as that in the last two Rakahs of Asr." (Sahih)